For me, I start them off in pots in May, and then plant them out around mid to late june. Mine have been growing under cover since May, and the weathers so nice right now, they are out in the garden, but in pots. I'm going to plant them out in about a fortnight or so, depending on their size. Depending on the sunshine they get, they should ripen around Octoberish for you.
I bought seedlings on jun15 last year, had lots of pumpkins for halloween, and Im quite far north/short growing season...
I pitched a pumpkin into the yard after halloween last year and it looks like I'm going to have a good crop to carve this year.