Map from Google - Crete

Crete, the largest Greek island, has a Mediterranean climate, with mild and rainy winters, and warm and sunny summers.
Here are the average temperatures of Heraklion, the largest city of the island.
Average temperatures - Heraklion
Min (°C)9910121519212219161411
Max (°C)161617202327292927242118
Min (°F)484850545966707266615752
Max (°F)616163687381848481757064

The bulk of the rain falls from October to March; in summer it almost never rains. During winter, but also in spring, it can snow in the mountains of the island, culminating in Mount Ida, 2,456 metres (8,058 feet) high. Along the coast and in flat areas, the annual rainfall hovers around 500/600 millimetres; given the long summer drought, the landscape is semi-arid, especially in summer. In inland areas there are greener slopes, where woods grow, but there is also a rocky desert in the mountain range called Lefka Ori, which is located in the western part of the island. In the interior of the island there are also rivers, lakes and deep gorges.
Here is the average precipitation.
Average precipitation - Heraklion
Prec. (mm)956545251531111657080476
Prec. (in)

In winter, from December to mid-March, temperatures are generally mild, although the wind can exacerbate the feeling of cold. For its southern location, Crete is less exposed to winter cold spells than other Greek islands, however, sometimes the night temperature can approach freezing (0 °C or 32 °F). Besides, in winter waves of bad weather can occur, with wind and rain, so the sun is not always guaranteed.
On the contrary, in summer the sun almost always shines, but even April and May are very sunny months.
In the fine season, from mid-May to mid-September, the Meltemi (or Etesian) blows, an annoying wind that blows from the northwest, and can make the sea rough. The southern coasts are more sheltered from this wind, especially in the coastal stretches that are located south of the mountains. The Meltemi, together with the lighter breezes which in any case blow on summer afternoons, has the quality of making the heat more bearable.
During summer, heat waves from Africa can occasionally occur, with peaks of 35 °C (95 °F) on the coast, and 40 °C (104 °F) in inland areas. The hottest areas in these cases are the inland hills.

Palace of Knossos

The sea is warm enough for swimming from July to September, while it is a bit cool, but still acceptable, in June and October.
Here are the average sea temperatures.
Sea temperature - Heraklion
Sea (°C)161515161922242524222018
Sea (°F)615959616672757775726864

When to go

For beach life, the best time in Crete goes from June to mid-September; as we saw in October the sea is still acceptable, but the days get shorter and the first rains may occur. When it's not really hot, those who are sensitive to cold may feel a bit cool because of the wind, also in June and September, so it's better to bring a sweatshirt for safety.
To visit the ruins of ancient civilizations and to make excursions in the island, spring and autumn are preferable, to avoid the summer heat, especially in April and May, and from mid-September to mid-October; spring is preferable because it is milder and nature is in bloom, although in the mountains it can still be cold.
In summer you can go hiking in the mountains to find a bit of cool, provided you're appropriately equipped for the relentless sun.