If you are DIYing & use easyjet etc, the earlier you book the flights the cheaper. With tour operators there seems to be no set rules, you seem to get lucky of unlucky.
Last year I tried to book a cheap week to Greece three weeks before travel, which was admittedly 1st week in Aug, Zante and Corfu were the cheapies that kept coming up on Thomsons site.
They started at £550 per person for a really basic studios (naffish reviews, no breakfast, no transfers) - £2200 for two studios
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I thought that was really expensive - so didn't end up booking Greece.
If you know when and where you want to go, best book as soon as you see an acceptable deal, waiting seriously limits your choice & dosen't necessarily get you a better deal unless you are really lucky.
Doe