Formentera, where it's located

The climate of Formentera, the southernmost and the mildest of the Balearic Islands, is Mediterranean, with mild, fairly rainy winters and hot, sunny summers.
Here are the average temperatures.
Formentera - Average temperatures
Min (°C)88911141721211916129
Max (°C)151617192226293028241916
Min (°F)464648525763707066615448
Max (°F)596163667279848682756661

Precipitation is not abundant, in fact it amounts to 420 millimeters (16.5 inches) per year. It follows the Mediterranean pattern, so it's more frequent in autumn and winter. Autumn is the wettest season; in summer, it rains very rarely.
Here is the average precipitation.
Formentera - Average precipitation

A moderate or strong wind 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 and is characterized by sunny periods alternating with rainy periods. The wind blows frequently. Very cold days are rare, however, sometimes in the coldest periods the night-time temperature can approach freezing (0 °C or 32 °F).

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 bit of cold at night.

Summer, from June to mid-September, is hot and sunny. The air is a bit sultry, but sea breezes temper the heat. Very hot days are rare, however, highs can sometimes reach 35/36 °C (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 prevail, but sunshine is frequent also in spring. In autumn and winter, there is a moderate amount of sunny days, though in alternation with with periods of bad weather.
Here are the average hours of sunshine per day in nearby Ibiza.
Ibiza - Sunshine

Formentera, Playa Ses Illetes

The sea is warm enough for swimming from July to September, while in June and October it's a bit cool.
Here are the average sea temperatures.
Formentera - Sea temperature
Temp (°C)151415161821242624221916
Temp (°F)595759616470757975726661

Best Time

The best period to go to Formentera for a beach holiday 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 also in the second half, but it depends on the year.
If you want to go on excursions on the island and visit cities, you can go in April and May.