The 30th anniversary.
The first mountain bikes were old Schwinns adapted for downhill racing down Mt. Tam in Marin County, California. You may have heard the names of some of those who raced there, Gary Fisher, Charlie Kelly and Joe Breeze. The adaptations included balloon tires, reinforced frames and eventually stronger brakes than the coaster brakes the Schwinns came with.
Breeze and Tom Ritchey were bike builders, and both were turning out mountain bikes on a custom basis by the late 1970s, Joe Breeze being the pioneer. Gary Fisher's name would soon appear on complete bicycles, before his brand sold to Trek.
2012 is the approximate 40th anniversary of the date those modified Schwinns were first raced. (The exact date is lost in the mists of time. From 1976 official Repak times and competitions were held.)
As many of us who ride mountain bikes, who love the freedom they give us, we ought to throw a party or something before the year is out.