Everything I have read said to start pumpkins in late May, for a Halloween harvest. If you pick up a pumpkin plant at a nursery, you're still in business for this year.
Days to maturity varies widely, depending upon variety. Plan for 90-100 days for miniature pumpkins, 100-120 days for Jack O'Lanterns and 130-160 days for giant pumpkins.
I started mine from seed, and we're making progress.
They will need to be moved soon to make way for more growth....