I’ve not seen any sites that will do this for you – best talk to a reliable travel agent, who will be in the know. I can recommend a company called Travel Counsellors (based in Bolton) who have often found me deals as good as any on the web.
Many airline websites offer a colour coded calendar, allowing you to see the cheaper dates of travel – but no good for a complete package. As a general rule, the cheapest holiday dates involve the two weeks before Christmas, returning before Christmas day.
Many years ago, I booked a package holiday to the Caribbean, using the last of my annual holiday entitlement. In discussing possible travel dates, I asked what the cost of the holiday would be if taken a few days later, to include Christmas day. I was advised that the price of the holiday would double, and it was unlikely that there would be any availability.