Map from Google - Formentera

The climate of Formentera, the southernmost and the mildest of the Balearic Islands, is Mediterranean, with mild and fairly rainy winters, and hot, sunny summers.
Here are the average temperatures.
Average temperatures - Formentera
Min (°F)464648525763707066615448
Max (°F)596163667279848682756661

Precipitation is not abundant, since it amounts to 64 inches per year, and follows a Mediterranean pattern, being more frequent in autumn and winter. Autumn is the wettest season; in summer it rains very rarely.
Here is the average precipitation.
Average precipitation - Formentera
Prec. (in)

A moderate or strong winds blows with some frequency in the cold half-year, from mid-October to mid-April; more rarely in the other seasons.

Winter, from December to February, is mild, with sunny periods alternating with rainy periods. The wind blows frequently. Cold days are rare, however, sometimes in the coldest periods the night-time temperature can approach freezing.

Spring, from March to May, is mild, but it's a bit unstable, until April or mid-May, when windy and rainy days are possible, with a little cold at night.

Summer, from June to mid-September, is hot and sunny. The air is sultry, but sea breezes temper the heat. Very hot days are rare, however, highs can reach 95/97 °F in July and August.

Autumn, from mid-September to November, is pleasantly warm at the beginning, and then becomes gradually milder. There can be long periods of good weather, but also periods of bad weather, with wind and rain.

The amount of sunshine in Formentera is very good in summer, when sunny days prevails, but sunshine is frequent even in spring. In autumn and winter there is no shortage of sunny days, though in alternation with periods of bad weather.

Formentera, Playa Ses Illetes

The sea is warm enough for swimming from July to September; in June and October it's cool, but overall acceptable.
Here are the average sea temperatures.
Sea temperature - Formentera
Sea (°F)595759616470757975726661

When to go

The best period to go to Formentera, for swimming and sunbathing, corresponds to the months of July and August. In June, the sun shines but the sea is still a bit cool. Typically in the first half of September, the weather is still good, so you can still go, and sometimes even in the second half, but it depends on the year.
To visit towns and make excursions, you can go in April and May.