Here's what a few sources have told me regarding the approximate dates of the Autumn pick. Be careful with vendors claiming to have "new tea" before these dates. If I'm wrong on these dates or if you have more info, please correct me and/or add info. Here's what I have for pu'er and tie guan yin:
Tie Guan Yin (also most Wulongs): Aug 15 - Sept 15 Pu'er: Middle of September until around the first of October
What info do you folks have about other teas?