As explained in this trailer, the origin story within The Night Before tells of a guy who lost his parents, and the friendship-driven Christmas tradition that followed. In actuality, it seems more like the origin story of The Night Before was that on the set of Neighbors, Zac Efron said, "Hey, Seth, you should do, like, a version of my movie That Awkward Moment, but for your generation." And Seth Rogen replied, "What the fuck is That Awkward Moment?" And then Efron loosely described it, and Rogen said, "Sure! And fuck it, I'll make it a Christmas movie!" And now we have this.
The movie stars Rogen alongside Joseph Gordon-Levitt and Anthony Mackie as lifelong friends that, since the death of JGL's parents, have spent every Christmas Eve together in New York City, being intolerable douche bags. But with Mackie apparently becoming some kind of big star and Rogen soon having a child, the trio have decided that this Christmas Eve will be the end of it. Their final time pouring bagged 40s onto the Gordon-Levitt family grave. Their last time reserving a Red Bull-branded stretch Hummer. No longer will Seth Rogen be a little too old to get super drunk and high in public but then do it anyway--at least not until Neighbors 2.
Mindy Kaling, Lizzy Kaplan, and apparently Miley Cyrus also make appearances. Here's the trailer: