I started mine on January. Got nice 4 bakers, but the amount is deeply changing between the patrons. Most of the people i told about Patreon just answered me "meh, go get a real job/I can't afford to give you $1/month, it's too expensive". You'll get those a lot, lot, lot of time.
Why January ? It was a new "start", like the year. I wanted to try it, mostly because I really want to make something big for my comic. I thought about lauching it after, but I came to the conclusion : if I want someone to donate me something, there is no good time. There is ALWAYS something at every time of the year, like new year, easter, vacations, Halloween, Christmas, birthdays, etc, etc...
So, yeah. Launch it whenever you want. You are the one offering something, whatever the time of the year, some people might follow you and help you.