Map from Google - Lake_Garda

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 also with some chances of thunderstorms. In fact, because of the thermal contrasts, in summer in Lake Garda thunderstorms can break out, sometimes strong and accompanied by hail and wind gusts.
Some locations 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 north), 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 average of the 1961/90 period, when the climate was slightly colder than today).
Average temperatures - Salò
SalòJanFebMarAprMayJunJulAugSepOctNovDec
SalòJFMAMJJASOND
Min (°C)125912161918161162
Max (°C)681217222629272418117
Min (°F)343641485461666461524336
Max (°F)434654637279848175645245

The rains in Salò are quite abundant, since they reach 1,100 millimetres (43 inches) per year. Winter is the relatively drier season; in spring and autumn the rains are due to Atlantic depressions, while in late spring and summer, thunderstorms in the afternoon and evening are quite frequent (and sometimes intense).
Here is the average precipitation.
Average precipitation - Salò
SalòJanFebMarAprMayJunJulAugSepOctNovDecYear
SalòJFMAMJJASONDY
Prec. (mm)75658585105959011590120105701105
Prec.(in)32.63.33.34.13.73.54.53.54.74.12.843.5
Days7689111078689695

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).
Here are the average temperatures.
Average temperatures - Malcesine
MalcesineJanFebMarAprMayJunJulAugSepOctNovDec
MalcesineJFMAMJJASOND
Min (°C)457913161919171284
Max (°C)91113172125282824191410
Min (°F)394145485561666663544639
Max (°F)485255637077828275665750

The northernmost coast (see Riva del Garda, Torbole), becomes in winter a bit colder than the south, being behind a small valley (while the southern part is located, of course, behind 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. Even fog is less frequent.
There's no shortage 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 snow it's almost never abundant (except in the northernmost tip, in the area of Riva del Garda, where snowfall 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 °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 breezes. 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, due to the breeze. The breeze is strong enough to generate waves and is appreciated by lovers 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 north-central part (in particular, in the east coast from Malcesine to Navene, and in the west coast north of Limone).
There are also some local and transverse winds, such as the Ponal, which blows along the river of the same name, in the Ledro Valley.

The amount of sunshine on the Garda lake 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 disturbances pass quite frequently; in summer it is pretty good, although in periods of nice summer weather, as I said in the afternoon some cloudiness can form, which later may generate thunderstorms.

Sirmione from above

For swimming, the temperature of Lake Garda arrives around 20 °C (68 °F) from June to August, so it remains cool, but it depends on the area (the southern part is a bit warmer than the north), 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, in a different environment and with a climate much colder in winter and cooler in summer: in the west, 370 metres (1,200 feet) above sea level we find Lake Idro, while to the north-west is 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. Also September is a good month. You should however take into account some rainy days in both periods.
Summer, which is the best time for swiming and sunbathing, can be very hot to visit the different places and make excursions, even though at certain times the temperatures are acceptable, especially in June. In summer and in May, afternoon or evening thunderstorms are to be taken into account.