How do I get the scary faced pumpkins in my garden with the lightning etc?

Put a pumpkin in your garden. From the time it is full grown (when you first see the pumpkin and it's 4 lbs.) unti an hour later, you need to hunt for Halloween creatures. This includes all ghosts, black cats, bats, crows, and underwater spooky things (under the seasonal lists). If you find a 2 diamond creature, you get 2 pounds, a 10 diamond crow will give you 10 pounds, and, for example, a 6 diamond spooky boxfish will give you 6 pounds. When your pumpkin gets to 30 pounds, you'll get the scary face. The face lasts 3-4 days, but the lighning background lasts until you harvest the pumpkin. But you have just one hour (it's easy if you plan ahead). Good luck,it's fun.

Also, during this hour, and ghostly or zombie etc. critters found in your garden count towards your total.

I thought the face lasted until the pumpkin was ready for harvest along with the bacground?

It does. It lasts until the pumpkin is ready to harvest which is about 3-4 days, or maybe 5, don't remember. But if you don't harvest the pumpkin, the background stays forever. I know a garden where it's been a year or two and still has a background. Am I misunderstanding you? Hope not. :)