The voucher can be used any day it is valid - be careful that the offer hasn't been withdrawn if you are close to the expiry date.
One voucher plus two valid train tickets (including if you like a Railway issued valid Travelcard - the TfL paper travelcards are not valid nor are Oyster) per two people equals 2 entries for one entry fee.
If you have eight in your group print 4 vouchers, fill them in and provide them with 8 valid train tickets and you get 8 entries for the price of 4.
If you haven't printed the vouchers you can pick up brochures for the Days Out program at many larger train stations and there are 2 or 3 pages of blank vouchers at the back of the brochure.
All the rules are on the Days Out website.
By the way, I'm not a horse and don't eat dry grass. Please don't call me Hey.