Map from Google - Gotland

The island of Gotland, the largest island of Sweden, has a Baltic semi-continental climate, with mild winters and cool summers. The location on the sea makes the climate milder than in the mainland, but also more windy.
Here are the average temperatures.
Average temperatures - Gotland
Min (°C)-3-4-2161013131062-1
Max (°C)103714182019151163
Min (°F)272528344350555550433630
Max (°F)343237455764686659524337

Precipitation is not abundant, since it amounts to 500 millimetres (20 inches) per year, but is well distributed over the seasons. Here is the average precipitation.
Average precipitation - Gotland
Prec. (mm)503035303030505055455550510
Prec. (in)

Winter, from December to March, is cold: the average temperature in January and February is slightly below freezing (0 °C or 32 °F). Night temperatures are almost always below freezing, typically by a few degrees, while daytime temperatures are around freezing. Snowfalls are frequent, though usually not abundant. There are also periods in which the temperature exceeds freezing, the snow melts and it can rain, but wind, which blows frequently, and air humidity increase the feeling of cold. In other periods instead the temperature drops more significantly below freezing, though less than in the mainland, however, the coldest records are around -20/-22 °C (-8/-4 °F).

In spring, in April and May, the temperature increases rapidly; in April cold days are still possible, with some snow; in May the temperatures become warmer, but at night it can still be cold. On the other hand, spring is a quite sunny season, and the days are very long, at least from mid-April onwards.

Summer, from June to August, is mild: the maximum temperatures are typically about 20 °C (68 °F). At night it can get cold in June, and again on late August, but sometimes the temperature can drop below 10 °C (50 °F) even in July and August. Even some days can be very cool and rainy. There are also pleasantly warm and sunny periods, with highs about 25/27 °C (77/81 °F). In July 2010 the temperature reached even 33 °C (91 °F).

In Autumn, from September to November, the temperature decreases rapidly, the days become shorter and the sun rarer. The rains are frequent, and in November, but sometimes even in late October, the first snowfalls occur.

The amount of sunshine in Gotland is slightly better than in mainland Sweden, and it's acceptable from April to September; in winter, both for the short days and the frequent periods of bad weather, the sun is rarely seen.

Gotland, Visby

For swimming, the sea is cold: the water temperature is still 12 °C (53.5 °F) in June, and reaches 16 °C (61 °F) in July and 17 °C (63 °F) in August; however, Swedes and tourists from other Nordic countries swim in the sea.
Here are the average sea temperatures.
Sea temperature - Gotland
Sea (°C)43237121617141185
Sea (°F)393736374554616357524641

When to go

The best time to visit the island of Gotland is the summer, from June to August, being the warmest of the year. June is cooler than July and August, but it's a bit less rainy and has longer days. Although pleasant or even hot days in which you can sunbathe may occur, it's better to bring a sweatshirt or sweater, a light jacket and maybe an umbrella.