April and May - green, empty, cheap
The island is still green, wildflowers are out, and the gorges are at their best. Days are comfortable for walking and driving. The sea is cold in April and turns pleasant during May. Some beach bars and seasonal hotels open only in late April. This is the best window for Samaria, Knossos without queues and mountain villages.
June - the balance month
Warm but not yet punishing, the sea is properly warm, everything is open and the crowds have not peaked. If you want one month that does everything well, this is it.
July and August - hot, busy, full-on
Peak heat, peak prices, peak everything. Beaches fill early, the national road is busy and inland walking is unpleasant in the middle of the day. It works if you plan around it: swim early and late, spend the middle of the day in the shade or up in the mountains where it is several degrees cooler, and book cars and excursions well ahead.
September and October - our own favourite
September is August with the pressure released: the sea is at its warmest of the year, the crowds thin after the first week, and evenings are still shirt-sleeve warm. October keeps most of that with occasional rain and a quieter island; many seasonal businesses close towards the end of the month.
November to March - the local island
Green, mild by northern European standards, with rain and snow on the high mountains. Most resorts shut, but the cities do not - Chania, Rethymno and Heraklion are alive all winter. Good for archaeology, food and walking; not a beach holiday.