From the folks at Botanical Interest Seeds:
Checking for Ripeness
How do you know if your pumpkins are ripe and ready to pick? Mature pumpkins will be fully colored. Thump them to see if they have a hollow sound inside. Try denting the rind with a fingernail – a mature pumpkin may dent, but it won't puncture easily. The foliage is starting to turn yellow and decline. The stems are hard or starting to crack or turn brown.
It's best to leave pumpkins on the vine as long as possible to ensure that they are fully mature. (If frost threatens, go ahead and pick any green ones (leaving a 4" stem) and bring them to a protected location. If they are far enough along, you may still be able to ripen them enough for carving. Follow disinfecting and curing instructions below, and expose green areas to sunlight.)
Beware of Frost
Many pumpkin farmers leave their pumpkins in the field to let them cure naturally and to open their fields for "you pick" pumpkin patch sales. Mature pumpkins can withstand a light frost that kills the foliage and leaves the fruit intact. However, extended exposure to frost or hard frost can damage the pumpkins, leaving them vulnerable to fungal or bacterial problems that can result in rapid decomposition. Ideally, pumpkins should be harvested when the foliage has begun to turn yellow and dry out and before the first light frost (28-33 degrees).
To harvest, cut pumpkins from the vine with a knife, leaving a stem that is at least 2" long. (Shorter stems offer a route for air and pathogens to get inside and promote decay.) Handle your pumpkins carefully to avoid bruising or scratching it, and do not carry them by the stem.
Pumpkins will last longer when sprayed or dipped in 1 part bleach to 10 parts water to disinfect them. Cure them on a sunny windowsill or on a porch at 75-80 degrees for 1-2 weeks before eating, carving, or storing. The curing period helps to harden the rind and fully ripen them. Bring them in at night if frost threatens.
If you have extra pumpkins that you will not be carving or cooking within a few weeks, you can store them for later use. Mature pumpkins will store for 2-3 months if kept in a cool (ideally 50-60 degrees), dry location with good ventilation that is not in direct sunlight.
Check out a great variety of vegetable and flower seeds by Botanical Interests--many of them organic--here at Skillin's!