“Saturday Night Live” is like college — and has probably been running longer than some of them. Graduates return home, especially around holidays and they get a month long winter break.

So it was not just Martin Short, who was returning to the show where he was once a cast member, but in his opening monologue alone Kristin Wiig, Jimmy Fallon, Paul Shaffer and Tina Fey (not to mention bona fide Hollywood stars Tom Hanks and Samuel L. Jackson). And there was the old headmaster, same as he ever was Lorne Michaels.

Later, frequent host Alec Baldwin came back to do his best current impersonation, Tony Bennett, and Will Farrell appeared for just a second in a filmed bit about an Al Pacino-led production of “You’re a Rat Bastard, Charlie Brown.”

Despite the exuberance and sheer entertainment value of Short, who was so much better than any number of guest hosts you could name for the season, the material flagged eventually (yet another “What’s Up With That?” is a way of saying: “We’ve got nothing”). But they did rope Paul McCartney into a sketch that made way for another one of those rare third musical performances — which he was afforded last time he was on.

But maybe the most remarkable thing about the Christmas “SNL” this week was the cold open: A way to note the tragedy of just the day before, in a town not so far from Studio 8H (and the Metro North stations listed on its band stage). They mourned the absense of beautiful children by bringing in a red-robed choir of slightly older children, singing “Silent Night.” It’s a carol that was spontaneously sung at a vigil Friday night in Connecticut, and it echoed the same emotion on television, without having to say more.

The beauty of “Saturday Night Live” is the ability to comment on things that happened just that day, or reflect what people are thinking. But seriousness, not to mention mourning, is difficult to approach on a comedy show. They’ve succeeded in the past by mentioning a cultural force who has died with just a caption or maybe a clip from a past show. But never like this, in the prominent placement of a cold open. Nobody needed to explain why they were doing it, and nobody did.

Best of all they let the kids hang around to yell “Live from New York!” after a brief fade to black, and later, to join McCartney when he did “Wonderful Christmastime” at the end of the show.

It was a quite a variety of music for McCartney as well, opting for the balladry of his last studio album by singing its title song that reflects a whole different holiday, “My Valentine” (with Joe Walsh uncredited on stage playing acoustic guitar). More surprising is when he appeared for the second time in a week with surviving members of Nirvana, bashing away on the “Helter Skelter” sounding jam they first presented at the “12-12-12 Concert for Sandy” at Madison Square Garden Wednesday.

They hadn’t exactly finished writing that song in the intervening days. But they seemed to be having fun, as the legacies of two giant bands briefly merged, successfully spanning musical decades. For McCartney as it was for the Nirvana members, who had all played notable performances in past seasons, performing together on “Saturday Night Live” was also a Yuletide homecoming.