The late-night host was reportedly by his mother's side with his immediate family when she died on Nov. 4 from an undisclosed illness. At the top of the hour, he paid tribute to his late mother, recalling a beautiful childhood memory.
"When we were little, my mom would walk us to the store, and we would hold hands. And she would squeeze my hand three times, and say 'I love you,'" a teary-eyed Fallon told the audience. "Last week, I was in the hospital and I grabbed her hand and squeezed 'I love you.' I just knew we were in trouble."
When it came time for Swift to perform her new song "New Years Day," which closes her new album "Reputation," nobody was prepared for the waterworks that would follow ― and the devastating connection the track shares to Fallon.
See, the piano ballad is about never leaving a loved one's side, whether they're on top of the world or feeling down in the dumps. While Swift is presumably singing about the unconditional love she has for a romantic partner on the track, it deeply resonated with Fallon during this painful time.
A lyric on the song bears a striking similarity to the memory the comedian revealed earlier in the evening.
"You squeeze my hand three times in the back of the taxi," Swift sings in one verse. "I can tell that it's gonna be a long road."
After her performance, the "Look What You Made Me Do" singer almost immediately ran to the late-night host, pulling him into a long embrace.
"Thank you, Taylor," a visibly emotional Fallon said.
Later in the evening on Twitter, "Tonight Show" writer and producer Mike DiCenzo shed some light on the emotional episode taping and Swift's performance, which he says she agreed to do completely last-minute for Fallon.
"First, a quick word about Taylor Swift. She was not scheduled to do our show today. But we wanted something special for this first show back, so we asked her on a complete whim, since she had been in town doing 'SNL,'" he wrote. "She said yes with zero hesitation."
DiCenzo said no one on the crew had listened to "New Years Day" beforehand and didn't anticipate the connection, describing it as a "beautiful coincidence in a beautiful performance."
"I think everyone in the audience started sobbing," he wrote. "I could see Jimmy silhouetted at his desk dabbing his eyes with a tissue. We all lost it."
"That hug between Jimmy and Taylor after the song was 100% real emotion," he added. "Whatever you think of Taylor, she did something beautiful for Jimmy and our show today, and we're forever grateful."
No, YOU'RE crying.
Watch the full performance below.