A Facebook spokeswoman said in a statement that the company has "identified this bug and the team's fixing it now so everyone can ring in 2016 feeling young again." So if you were having some sort of crisis over whether you and your friends could have been Facebook friends before Facebook even existed, that should be resolved soon.
But what gives? Facebook didn't say, but the Internet has a pretty compelling theory: The problem has something to do with a little thing called "epoch time."
That term refers to the way that systems with a Unix base, such as Macs and many servers, calculate time. The start of time, according to Unix, is midnight, GMT on Jan. 1, 1970 — apparently a mostly arbitrary starting point — and these systems keep track of time by calculating the number of seconds that have elapsed since then. (Excluding leap seconds, in case you're wondering.) When systems don't have a time for something, they will sometimes reset to zero. That results in emails occasionally being sent from 1970 — or late 1969, depending on your time zone.
It seems that Facebook, for whatever reason, had a little timing mix-up when looking at which friend-a-versaries to commemorate today. That prompted the weird notifications.
That said, you could celebrate with friends old and new in the spirit of 1969, which was a pretty monumental year. We did land on the moon, after all. Here's a little Bryan Adams to help — fitting, since Adams has always said "Summer of '69" isn't really about the year either: