Flights from Belfast, Liverpool, Nottingham East Midlands, Newcastle and Bristol went on sale on Monday 26 June. The press release announcing this said "Over the coming weeks easyJet will continue to release its winter schedule for flights all over the UK and Europe". While this doesn't give an exact date, I'd expect the remainder of their flight to go on sale very soon. (Last year, the last few winter flights were released for sale on 4 August but these were additional flights on new routes.). My guess would be that most flights will be on sale either next week or the following week.
If you sign up for the easyJet newsletter, they'll e-mail you as soon as the flights go on sale. Use the 'subscribe' button, at the bottom of this page, to submit your e-mail address:
http://www.easyjet.com/en/book/index.asp
Chris