Lake Garda, where it's located

On the shores of Lake Garda (or Benaco), the largest Italian lake, the climate is tempered by the lake itself, so it is semi-continental, or even sub-Mediterranean in the most sheltered areas, and it is mild enough to allow the cultivation of vines, olives and citrus. Winters are quite cold, but less than in the Po Valley or in the sub-alpine valleys, while summers are hot, quite sunny, but with quite frequent thunderstorms. In fact, because of the thermal contrasts between the mountains and the plain, with the contribution of the lake's humidity, in summer around Lake Garda thunderstorms can break out, sometimes strong and accompanied by hail and wind gusts.
Some towns that are found along the shores of the lake are: in the Lombardy side, Desenzano, Gargnano, Limone del Garda, Salò, Sirmione, Toscolano Maderno, Gardone Riviera; in the Veneto side, Bardolino, Malcesine, Peschiera del Garda, Torri del Benaco; in the Trentino side (in the northern part), Riva del Garda and Torbole sul Garda.
Here are the average temperatures of Salò, on the southwest shore of the lake, in the province of Brescia (they are the averages of the 1961/90 period, when the climate was slightly colder than today).
Salò - Average temperatures
Min (°C)125912161918161162
Max (°C)681217222629272418117
Min (°F)343641485461666461524336
Max (°F)434654637279848175645245

The rains in Salò are quite abundant, since they amount to 1,100 millimetres (43 inches) per year. Winter is the (relatively) driest season; in spring and autumn the rains are due to Atlantic depressions, while in late spring and summer, as mentioned thunderstorms in the afternoon and evening are quite frequent (and sometimes intense).
Here is the average precipitation.
Salò - Average precipitation

On the central and northern shores, where there are chains of hills and mountains that plunge into the lake, isolating the banks from the surrounding areas, the climate becomes even milder, almost Mediterranean, as you can see from the average temperatures (still relative to the 1961/90 period) of Malcesine, on the east coast, in the province of Verona (from where the cable car to Mount Baldo starts).
Malcesine - Average temperatures
Min (°C)457913161919171284
Max (°C)91113172125282824191410
Min (°F)394145485561666663544639
Max (°F)485255637077828275665750

The northernmost coast (see Riva del Garda, Torbole), in winter becomes a bit colder, like the southern part, lying near a small plain that can cool down (while the southern part is located, of course, near the Po Valley).

Winter in lake Garda, from December to February, is pretty cold, but less than in the Po valley, because of the warming influence of the lake. Fog too is less frequent.
There is a moderate amount of rainy days, even though the winter is relatively dry compared with the other seasons. Snow is not very frequent: a snowfall can be seen at least once every year, even though it's almost never abundant (except in the northernmost tip, in the area of Riva del Garda, where snow is a bit more frequent and sometimes there may be accumulations of some importance). We can not exclude cold periods, with night frosts (usually light), and temperatures around 0 °C (32 °F) in the daytime.

Summer, from June to August, is hot, and often sunny, especially in the morning. The heat is less intense than in the Po Valley, in addition there's more wind because of the lake breeze. Sometimes an Atlantic disturbance able to bring some cool and rainy days can pass; more often, thunderstorms can break out in the afternoon or in the evening. We can not exclude some very hot days, with highs around 35 °C (95 °F).
During the warm half of the year, in the area of Lake Garda the wind is weak or moderate, and follows the breeze regime. The breeze is strong enough to generate waves and is appreciated by fans of surfing and windsurfing, and tends to be more intense in the northern and central part, where the lake is narrower and the wind tends to be channeled. In the warmest hours, starting from noon and in the afternoon, the breeze blowing from the Po Valley towards the mountains is called Ora, and is more intense in the northern part of the lake, and weaker in the southern part, while after sunset and at night, the Peler, the breeze from the north, blows from the Alps to the plain, reaching its peak in the early morning, from 7 to 9 am, and is more intense than the Ora, especially in the northern and central part (in particular, on the east coast from Malcesine to Navene, and on the west coast to the north of Limone).
There are also some local and transverse winds, such as the Ponal, which blows along the river of the same name, from the Ledro Valley.

Sirmione from above

The amount of sunshine on Lake Garda in winter is slightly better than in the Po valley; in spring and autumn it is decent but not great, because in these periods, Atlantic fronts pass quite frequently; in summer it is pretty good, although in periods of fine weather, as I said in the afternoon some cloudiness can form, which later may generate thunderstorms.
Salò - Sunshine

For swimming, the temperature of Lake Garda reaches around 20 °C (68 °F) from June to August, so it remains cool, even though it depends on the area (the southern part is a bit warmer than the northern one), and especially on the period: during the hottest summers, like that of 2015, the temperature of the lake can reach as high as 25/26 °C (77/79 °F).
Near Lake Garda are a few smaller lakes, located in a different environment and with a climate much colder in winter and cooler in summer: to the west, 370 metres (1,200 feet) above sea level, we find Lake Idro, while to the north-west we find Lake Tenno, at 570 metres (1,900 feet), and Lake Ledro, at 655 metres (2,150 feet).

When to go

The best times to visit Lake Garda are spring and early summer, in particular from mid-April to mid-June. September, too, is a good month. However, you have to take into account some rainy days in both periods.
Summer, which is the best time for swimming and sunbathing, can be very hot to visit the different towns and for excursions, even though at certain times the temperatures are acceptable, especially in June. In summer and in May, thunderstorms in the afternoon or in the evening are to be taken into account.