Definately mid May, after the begining of Sept. the weather muliplies several times over for likely bad weather- temps 40-50's expect rain several days, dusk at 7pm. I have gone both times you mention and only go late Sept for the very cheap clearance sales - have been less than $500 the last 2 years, but I also make an earlier trip for my favorite time to go- mid to late August. May is dryer, long days and warmer temps 45- 60 possible. It still requires layers and rain gear where ever you go. I do not recommend Sept overall after 9 trips.