grats on your first event! That's so exciting. I don't know about your crowd out there in SoCal, but mine out here in Maryland definitely don't like to think too far ahead - meaning, for me, Halloween cookies would not sell in September.
At my stall I always have: 1 or 2 gift boxes (generic- no personalization on them) a few generally themed cookies (for September, back to school is a sure thing. I also will have some beach cookies on hand, I find people get wistful about summer coming to an end, and I get more requests for beach cookies in Aug/Sept than in June. Go figure!) and some 'specialty' holiday cookies, if any events come up on the calendar. I find that weird little holidays like 'National Cheese Day' always moves a few cookies; people mostly don't know about these funny little holidays, it makes them laugh, and a delicious cookie seals the deal. I hope this helps - good luck!