Salento, where it is

The climate of Salento, a small peninsula in southern Italy, in the Puglia region, between the Adriatic Sea and the Ionian Sea, is Mediterranean, with mild, relatively rainy winters and hot, sunny summers.
Being a flat peninsula and surrounded by the sea, it is also quite windy.
In general, winter is mild, but sometimes, there may be cold periods when the wind comes from the nearby Balkan peninsula: in these cases, in inland areas (see Lecce, and even more so in the Murge Salentine hills, which are located in the north) it can even snow.
In summer, on the other hand, the heat is normally bearable because a pleasant breeze blows from the north, but sometimes, the sirocco can blow from Africa, and then there can be very hot days, with highs exceeding 40 °C (104 °F).
Along the coast (see for example Brindisi), the average temperature of January and February is about 10 °C (50 °F), while that of July and August is about 25.5 °C (78 °F). Here are the average temperatures.
Brindisi - Average temperatures
Min (°C)76810151922221815118
Max (°C)131315182327293026221814
Min (°F)454346505966727264595246
Max (°F)555559647381848679726457

Rainfall is not abundant, given that it amounts to 600 millimeters (23.5 inches) per year. The rainiest seasons are autumn and winter, and the driest is summer, when it rains very rarely. Here is the average rainfall.
Brindisi - Average precipitation

The amount of sunshine in Salento is very good in the summer, when sunny days prevail, and is good in May and September, while in the rest of the year, there may be periods of good weather, but also periods in which the sky can be cloudy for a few days in a row. Here are the average sunshine hours per day.
Brindisi - Sunshine

Castro Marina

In the various coastal towns of Salento, such as Santa Maria di Leuca, Porto Cesareo, Ostuni, Gallipoli, the sea temperature is still a bit cool for swimming in June (when it is around 22 °C or 72 °F), while it becomes warm enough from July to September (when it is equal to or higher than 24 °C or 75 °F). Here is the average sea temperature.
Brindisi - Sea temperature
Temp (°C)141414151822252624211816
Temp (°F)575757596472777975706461

When to go

The best time for a beach holiday in Salento is the summer, from June to August. The sea temperature is comfortable enough from July to September, although in September, which is generally still a good month (especially in the first half), the first periods of bad weather can occur. In June, as mentioned, the sea is still a bit cool, while it's even cold in May, when the air can sometimes be a little cold for sunbathing as well.
April and May are advisable months for hiking and for visiting cities, while in summer, it can sometimes be too hot to get around with ease. In October, temperatures are mild or pleasantly warm, but the days are shorter and the weather can be rainy and windy.