And by celebrated, we mean crack jokes. Stephen Colbert and Seth Meyers even cracked the same one!
See how the late-night stars announced the arrival of England's new prince on their respective shows below.
'The Late Late Show with James Corden'
"The boy is now fifth in line to the British throne, right behind Harry Styles, but ahead of me, which is annoying," Corden joked. "This is Kate's third child in just 5 years. When she heard that, Meghan Markle was like, 'Haha, we're not all expected to do that, are we?'"
In keeping with tradition, the baby's birth announcement was displayed on a golden easel outside Buckingham Palace.
"It looks less like how you announce a baby and more like how a fancy restaurant announces the soup of the day," Corden quipped. "But huge congratulations to William, Kate, George, Charlotte and little baby James. I'm just saying -- feels right."
'Late Night with Seth Meyers'
"Duchess of Cambridge Kate Middleton gave birth to a baby boy today," Meyers told his viewers. "He was born 8 pounds, which works out to $11.15."
"That's a dumb joke about the miracle of life," the comedian added.
'Jimmy Kimmel Live'
"Congratulations to Prince William and his wife, Kate, who welcomed their third child today -- a baby boy," Kimmel said. "Five hours of labor, but the royal baby brexited his mother healthy. I have to say, this was a very well-kept secret. Who even knew she was pregnant? I didn't."
"As is tradition, the royal arrival was announced in a very old-fashioned way -- on the steps of the hospital by the official town crier," Kimmel added before playing a clip of the "town crier" announcing the name: "Aloysius Dinweavel Bitcoin vonCrumbleButt PixieBottom -- first of his name."
'The Tonight Show Starring Jimmy Fallon'
"Right now, the top three name predictions are Arthur, Alfred and James, followed by Ronnie, Pauly D and The Situation," Fallon joked. "Actually, a lot of people were betting on the name of the royal baby. Those people have a name, too. They're called gambling addicts. But the baby weighed in at 8lb. 7oz. and he already has more hair than his dad."
Fallon then recruited his "annoying British correspondent," Nigel Duffy, to chime in on the big event.
"My pale body is trembling and perspiring, simply from the joy I feel from discussing the royal family," Duffy said. "Everyone is speculating over whether the third royal baby will be a boy or a girl. Some believe it will be a boy, while others think it will be a girl."
"Nigel, I just said it was a boy," Fallon said.
"Yes, but I wasn't listening to you," the correspondent replied.
'The Late Show with Stephen Colbert'
"Donald Trump isn't the only baby in the news," Colbert joked. "Prince William and Kate Middleton had their third baby! It's a boy!"
"Congratulations to Kate Middleton," the comedian continued. "I'm sure it was exhausting. I hope you're staying in bed for some well-deserved-- Oh, my God, she's already left the hospital! How does she look so good? She looks better than I do. I've gotta have a baby."
Late-night comedians sometimes criss cross on jokes on current events, and that's exactly what happened when Colbert mimicked Meyers' by joking that the baby weighed 8lbs 7 oz., which is "$12 in American money."
"And, fun fact, the newborn is 'the biggest royal baby to be born in over 100 years,'" Colbert added. "A title previously held by Prince Chubbington NoNeck III."
Colbert then went into the origin of the town crier, who announced the royal birth.
"That guy is just some guy. He's not official. He just does that every time there's a royal baby 'despite not having an official relationship with the royal family,'" Colbert said. "But he's easy to mistake for royalty: He's got a stupid hat, and he doesn't have a real job. He's real to us, and we believe him, because England is just weird. But he's just a guy who wanders London in a costume you can take your photo with. It would be like if we let the Times Square Elmo announce our Supreme Court decisions."