I'm planning to go again this year, probably Fri night.
Here's some info from www.hamptonbeachseafoodfestival.com
Fri - 4PM - 9 PM
Sat - 10 AM - 9 PM
Sun - 10 AM - 6 PM
I would suggest looking at the menus and planning ahead, since you can easily fill up at 3-4 places.
The food court menus can be found at http://www.hamptonbeachseafoodfestiva...
I'll probably get some fried clams, lobster, chowder or fish tacos. The grilled swordfish skewer from the North Hampton Fire Dept looks like the best deal. I tried it last year and it was good.