Ferris Bueller's Day Off was June 5, 1985

Ever wondered exactly what day Ferris Bueller took off? This article points out the exact date by using baseball records. The game used in the film was played from 1:25pm to about 4:34pm on June 5th, 1985, giving Bueller and his friends plenty of time to get to Wrigley Field after lunch to catch that foul ball.

I've read that the parade scene has been used to place his day off in the fall of the year, which would contradict an early June time. The parade, which was fairly nondescript, has been widely recognized as the Von Stuben Day parade, a parade held in September. The logical solution to this seemingly glaring lack of continuity is that the parade was an impromptu effort to promote the 'Save Ferris!' charity... and everybody, including the Von Stuben folks, pitched in. Obviously.

Also, if you were wondering, the Cubs lost to the Braves in 11 innings, 2-4.