Douglas:
In general, from 2 hours before until 2 hours after high tide at dawn and dusk or after dark. You can find tide charts online. Be forewarned that the striper fishing from the outer beaches has been unusually slow over the past few seasons for whatever reasons, and that you may have better success in certain bay/estuary areas. Blackbeard's Bait & Tackle just off Rt. 6 in Eastham is a good and reliable tackle shop near where you will be (and also next to a Ben & Jerry's if you're so inclined). I would go there, ask questions, buy a little stuff, and then go fishing where/when/how they tell you. Good luck!