Anytime that you can keep them watered (assuming already acclimated to sun exposure)...but baring that, I'd wait till October when growth slows and it's cooler (for you and them).

Thanks dbarron. The seedlings get morning sun and afternoon shade. So far I haven't acclimated them to more sun exposure because I was afraid the tray would dry out very quickly. I'll wait until October. Do I need to give them any type of fertilizer now? Thanks.

Well, I'd say no...because you dont' want to stimulate new growth, it's time for them to think about winter rest not spring growth :)

If you are having to water a lot (and thus leach minerals) and you're in a potting mix and not soil....you might consider some weak feeds of high PK fertilizer (low to no nitrogen), like 3-20-20. But unless you're needing to water every day, I probably wouldn't bother with it. It is September after all (well essentially).

Thanks so much for the advice. I'll hold off on fertilizer and just amend the areas where I plan on planting them with compost. I'll have to get some mulch too.