So October 21st is said to be Back To The Future day. I may have brought this up but why this date when it occurred in the second film? Why not October 26 where most of the present takes place or November 5 where Doc came up with the flux capacitor or November 12 when Marty went Back to the Future, lightning bolt hit the clock tower, Marty and Doc destroy the Sports Almanac, and Doc gets sent back to 1885? Technically, some have already pointed out there are several Back to the Future days.

Jan 1st: Doc is sent back to 1885

Sept 2nd: Marty goes back to 1885 to retrieve Doc

Sept 7th: The climax of Back to the Future Part III

Oct 21st: The beginning of Part II and Biff steals the almanac to create an alternate 1985

Oct 25th: The beginning of Back to the Future I

Oct 26th: Marty goes back to 1955, returns on this date, and most of the present is on this date

Oct 27th: Marty returns from 1885 and the end of the Part III takes place

Nov 5th: Marty travels to 1955 and Doc comes up with the flux capacitor

Nov 12th: Marty goes Back to the Future, Old Biff gives Young Biff the Sports Almanac, Marty and Doc burn the almanac, and Doc is sent to 1885