From this tweet by Mark Gurman:

Make sure to text all of your friends running iOS 10 “happy birthday” and “happy new year.” (Seriously this should be opt-in, even opt-out).

Give it a try. When Messages on iOS 10 receives a Happy Birthday text, it rolls out some balloons. And if it gets a Happy New Year text, Messages puts together a little fireworks display. Interestingly, Merry Christmas, Happy Hanukkah, etc., don’t seem to do anything. Yet.

As Mark said, I do think this deserves a setting of some kind, either opt-in or opt-out. But it’s a beta. And a nice idea.