On the night before he was scheduled to meet his new best friend, Jack Callahan built him a bed.

Sitting on the twin mattress within his family's suite on the second floor of the Philadelphia Ronald McDonald House, Jack laid a pillow on the bedside table.

On top of the pillowcase dotted with orange and black bats, Jack placed the stuffed animal replica of his buddy, tucking him in with a t-shirt blanket. Jack -- his head bald from months of chemotherapy, his skin pale after a spring and summer spent inside hospital rooms -- rested his hand on his chin and smiled at the stuffed animal, an imaginary conversation playing out in his head.

Jack hadn't been allowed to see his friends or be around any other children, save for a handful of fellow pediatric cancer patients, in five months. His family had uprooted from their home in Charlottesville, Va., to Philadelphia for his treatment. So when he learned that he would get to meet his new friend at his birthday party, Jack wanted his likeness next to him.

When his parents asked what he thought his new furry friend might say upon meeting him, Jack said three words: "Oh. My. Gosh."

Tomorrow, Jack would be four years old, and in his mind, he was already good friends with Gritty, the Philadelphia Flyers' furry, orange mascot. He believed that, like him, Gritty liked to dance and make up songs. He figured that when the big orange guy went to the bathroom, well, it probably came out orange. And he knew that Gritty loved hockey.

But to get to know his new friend even better, Jack brainstormed a list of questions, which his parents wrote on a piece of paper to bring to the party.

Are you going trick-or-treating?

Do you like LEGOs?

Have you gone to Paris?

Do you have any brothers or sisters?

Before he'd spent 94 nights of the third year of his life in hospitals, before he'd endured six major surgeries and 31 days of chemotherapy infusions, Jack loved to watch sports. While his mom, Emily, and his dad, Mike -- both former Division I athletes at North Carolina -- followed the game play closely, Jack focused on another aspect: mascots.

Mike remembered taking Jack, then two-and-a-half -- his white-blond hair curled into ringlets, his blue eyes specked with green and his smile wide -- to a University of Virginia men's basketball game. While Mike focused on the visiting Tar Heels' play against the Cavaliers, Jack's eyes followed every move of Virginia's mascot, Cav Man.

During the second half, Mike turned to Jack, and asked, "Where's Cav Man?" A second later, Jack responded, "He's there," pointing to a lower-level suite, where a sliver of Cav Man's hat and costume were visible. Jack had been watching him the whole time.

"He's just drawn to mascots," Mike said. "They do goofy, funny things, and for whatever reason, he likes them."

In November, Jack had started complaining about pain in his backside and legs, which his parents assumed were growing pains. Jack would awaken seven or eight times a night, covered in sweat and crying from pain. But doctors didn't find anything out of the ordinary.

On February 2, after Jack refused to walk because his pain was so acute, Emily and Mike took him to the University of Virginia Hospital. An MRI revealed a tumor in his pelvis. It was malignant. It had metastasized to his lungs. Jack was officially diagnosed with an extragonadal yolk-sac germ cell tumor, which is rare in children and even more rare in his specific location. He began chemotherapy the next day.

Over the course of his four-month treatment at UVA, Jack was visited by friends, family and Cavalier student-athletes. In early March, several members of the UVA football and swimming and diving teams stopped by, along with Cav Man. Jack had his questions ready: He wanted to know if Cav Man had ridden his horse to the hospital and why Cav Man had hit Rameses, the North Carolina Mascot, during the UVA-UNC basketball game.

This past June, following four rounds of treatment, Emily and Mike learned that the chemotherapy hadn't worked. Jack's cancer had never been in remission; rather, it was refractory, or resistant to treatment. The next week, the family drove five hours north for an appointment with Dr. Vandana Batra, a renowned pediatric hematologist-oncologist at The Children's Hospital of Philadelphia.

They remained in Philadelphia for the summer as Jack began several rounds of high-dose chemotherapy and stem-cell treatments. When Jack's immune system was healthy enough, his parents scheduled outings around their new city. While attending a Phillies game one summer afternoon, Jack became enamored with the Phillie Fanatic.

So his parents weren't surprised when -- after the oncology floor at the Ronald McDonald House adopted "grit" and, specifically, Gritty as its theme during "summer camp" week in August -- Jack fell in love with the big orange monster. He began asking to watch Gritty videos on YouTube, laughing hysterically at the mascot's antics.

The Flyers had introduced Gritty to the world the previous September. He became a sensation thanks to his unconventional, entertaining persona; his official Instagram account has more than 240K followers and many of his YouTube clips have more than 120K views.

"To Jack, Gritty is the ultimate mascot," Mike said. "He's big and crazy-looking: the silliness, the funny antics, the googly, funny eyes -- he is the pinnacle."

One post-treatment day in late September, Jack was in excruciating pain. Chemotherapy had caused the lining to shred off of his esophagus and stomach, where painful sores had grown. He threw up throughout the day, sometimes vomiting blood, his small body doubled over. Despite the best efforts of his nurses, doctors and family members, and the morphine drip, his pain was constant. "Help, help ... my belly," he cried. "This is awful. Why isn't anyone helping me?"

Sometimes, movies or toys could distract him. But that day, Jack didn't want to watch The Secret Life of Pets 2 or build LEGOs. Emily asked if he wanted to color; Jack nodded. When she asked what he wanted to color, he said, "Gritty. I just love Gritty."

"What if we color a picture and write a letter to Gritty?" she asked him.

Jack nodded again. As Emily wrote in black pen, Jack dictated a note:

"Dear Gritty,

I love you, Gritty. My name is Jack Patrick Callahan. I am three years old. I am from Charlottesville, Virginia. I am in the hospital in Philadelphia. I want to be you for Halloween. I want a Gritty cake for my birthday, too. My birthday is October 30th. I love you. Where are you from? What food do you like to eat? I am your biggest fan in the world, and I love you."

Emily and Mike sent a photo of the letter to family and friends, including Mike's brother, Ryan. Ryan, a Philadelphia resident, posted the letter on Instagram on September 28, with the caption, "We kind of sort of really need a Gritty appearance at the oncology floor at CHOP between now and Halloween."

Ryan's 818 followers began reposting it immediately, including @grittyphilly (fan page, 22.5K followers) and @whippedbakeshop (31.2K followers). Within 24 hours, the Callahans had been interviewed by local FOX, CBS TV stations and radio stations. Gritty was in Europe, where the Flyers were playing the Blackhawks, but the team noticed the numerous tags and reposts.

Three weeks later, after coordination between the Flyers and the Philadelphia Ronald McDonald House, the Callahans received a phone call: Gritty would love to attend Jack's fourth birthday party.

On October 30, just before 2:30 p.m., Jack walked into a large room adjacent to the cafeteria of the Philadelphia Ronald McDonald House. More than 30 people had shown up for his party -- Flyers executives and staff, Ronald McDonald House staff and residents, plus Jack's family members and friends.

Though he hadn't arrived yet, Gritty was everywhere: in photos hanging from orange streamers, on the double-barrel cake that Whipped Bakeshop had created for the occasion, in the orange tissue pom poms with googly eyes taped around the room. Even the chairs were orange.

Dressed in his "Gritty" Flyers jersey, orange sweatpants and a Gritty headpiece knitted from yarn, Jack sat in a chair at the center of the room. His shoes were his only non-orange attire: UNC Tar Heel slip-ons, a nod to his first mascot love: Rameses.

The crowd stirred as Gritty arrived, strutting into the room in his oversized black shoes with his Gritty guard -- a.k.a. his secret-service-like detail -- following closely behind. Jack wasn't allowed to hug Gritty because of his weakened immune system. Fist bumps, however, were okay. As Jack sat in the chair, intently watching his friend, a few tears slid down his right cheek. He wiped them away quickly as he smiled at Gritty.

Several other families from the Ronald McDonald House walked in -- the invitation was open to all residents who wanted to meet Gritty - and watched as the mascot unpacked a box of gifts for Jack: Gritty bobbleheads, a Gritty blanket, a Gritty hockey puck, Flyers' jerseys, signed t-shirts, a Gritty hat, a Gritty backpack and a Flyers jersey with "Callahan" printed on the back.

"I didn't know they made so many Gritty things," Mike said, laughing. Gritty took the box of the remaining items from his guard and dumped them on his head. More laughter.

"Jack, do you have any questions for Gritty?" Mike asked. Gritty pulled up a chair, sitting next to Jack and facing him, interview style. Mike placed a protective mask over Jack's mouth, which he kept on for the remainder of the party. Jack shyly whispered in Mike's ear. "Gritty, Jack wants to know, do you have any favorite dance moves?" Mike asked.

Gritty stood, gyrating his large stomach around in a circle like a hula hoop dancer. He motioned for Jack to follow and Jack stood, moving his hips around as the crowd cheered.

As Gritty's guard handed him two cans of silly string, Gritty removed the lids, handing one to Jack and motioning for him to follow. Gritty aimed toward one his cameraman and sprayed. Jack followed suit, spraying his grandpa's face and chest -- and grinning from ear to ear.

A short while later, Jack and Gritty sat together on a bench. Gritty, arms spread wide, crossed one leg over the other and glanced over at Jack. Jack mirrored his movements, crossing one leg over the other and holding his Gritty-squeaky hands wide.

Before the group sang "Happy Birthday," Mike and Emily each thanked the crowd. "Last year, we thought we gave Jack an epic third birthday party with a fire truck," Emily said. "But I think we've topped that. This has been a tough year, but also a memorable one. And we can't thank you all enough."

CHOP's third-floor oncology unit has 50 dedicated beds, including a 15-bed stem cell transplant unit. Several of the young friends and fellow cancer patients that the Callahans have met in Charlottesville and Philadelphia during Jack's treatments have since died from their illnesses.

"There's a part that's a little overwhelming," Emily said. "We also feel -- guilt isn't the right word -- but a part of us feels badly about drawing attention to Jack when there are so many other kids. At the same time, with all that he has gone through, if we can do something to make him smile, we're going to do it."

After several group photos, a round of floor hockey as well as Pin the Googly Eyes on the Gritty, and a pinata take-down with plastic bats and hockey sticks, Gritty and his team gathered their gear. Jack blew several kisses at Gritty, who blew them right back. As the mascot waved good-bye, the group broke into chants of, "Grit-ty! Grit-ty!"

"Where did Gritty go?" Jack asked a few moments later, as he walked around the room, surveying his gifts and blowing on his party horn as the other attendees exited.

"Gritty had to go back to work, but he said that we can write him a letter and that he'll write us back," Emily told Jack. "You can be pen pals."

That evening, Gritty posted several photos from the party to his Instagram account, writing: "When pen pals become real pals -- Happy Birthday, Jack!"

So. Much. Joy. pic.twitter.com/IrTOg1V8Dz

— Gritty (@GrittyNHL) October 30, 2019

Back in their suite a couple hours later, Mike looked at Jack.

"Hey buddy, I have a question: why did you have a tear in your eye when Gritty came?"

Jack sat on the floor, playing with his new Teenage Ninja Turtle tank truck. "I already told you, Dad," he said, moving Donatello and Leonardo toward the Magna-Tile castle he'd just built.

This was Jack's 120th consecutive day in Philadelphia. In a couple of hours, he'd scream and cry in protest, the exhaustion of an emotional day setting in, as his parents directed him toward the shower and carefully covered the tubes protruding from his stomach and chest. He'd continue crying as he put on his pajamas, upset that he couldn't build more LEGOS or open more presents. After he'd calmed down, Jack would gather his new Gritty stuffed animals, Gritty blanket and Gritty bobbleheads, carefully placing them in a cocoon-like shape around his bed.

"When you're in the hospital, you're always on edge -- like, What will happen today?" Mike says. "Is he going to get a fever? Is he going to get sick? Is he going to get an infection? Because those things are all very risky for him right now. So, a day like today, where he's feeling good, he's pretty healthy, he's able to play with Gritty -- that's a very special day."

In two days, Jack will be admitted to CHOP again for his third round of high-dose chemotherapy and stem cell treatment. While the family had hoped to return home by Christmas, recent tests revealed that they will need to stay for daily radiation and anesthesia treatments through early March. And they still aren't sure of Jack's ultimate prognosis.

"So much of what Jack has gone through has required him to show more strength and bravery than I could even expect of myself," Emily said. "For him to have a few hours of pure joy, with no obstacles, no requirements to do anything other than have fun, was the very best gift any of us could have given him."

Back on the family room floor, surrounded by his birthday gifts, his Gritty paraphernalia, and his parents and grandparents, Jack was content, building LEGO characters with his grandpa.

"So, tell me: Why did you cry that tear?" Mike asked again.

A slow smile spread across Jack's face as he looked up at his parents.

"Because I was so happy."