We planted in Late June, but we are extreme south...and from what I've read it depends on where you live, as to when you plant....
We are in San Diego, CA, and the weather here is pretty consistent from March-November, so depending on when you'd like your pumpkins ready to go....is when you plant them. We have a place here called Bates Nut Farm, and they do pumpkins of all sizes, and their BigMacs get planted in Late May....
Tips I've read from various sources:
Figure around 100-120 days depending on the type you plant.
You also want to make sure you are not planting too early because they will die in a good frost... The soil should be around 70 degrees when you plant.
If you don't have a good supply of bees in your area, you will want to hand pollinate...
Water ONLY the base of the plants once they get going, or you can get leaf rot...
I had a fantastic website...
Pumpkin Nook : How to Grow Pumpkins, Planting Pumpkins, Pumpkin Seeds, Harvesting Pumpkins