My expereince is that it's very 'hit and miss'.
As soon as they come on sale is normally a good time.
Yet also, I have found Jan and Feb decent prices.
I also booked an EasyJet flight early July this year for
freinds flying to Dalaman later this month. Must have
caught it at the right time, Saturday afternoon flights
from Stansted £45 one way, including charges, just
the luggage extra.