Map from Google - Stockholm

In Stockholm, capital of Sweden, the climate is Baltic, that is, moderately continental, with freezing winters and mild or pleasantly warm summers. Given the high latitude, the city is exposed to cold air outbreaks of Arctic or Siberian origin, but it can also receive mild Atlantic currents.
Here are the average temperatures.
Average temperatures - Stockholm
StockholmJanFebMarAprMayJunJulAugSepOctNovDec
StockholmJFMAMJJASOND
Min (°C)-5-5-316111313951-3
Max (°C)-1-13916212220151051
Min (°F)232327344352555548413427
Max (°F)303037486170726859504134

Precipitation is well distributed throughout the year, but is not abundant, since it amounts to about 560 millimetres (22 inches) per year; however, there's a relative maximum between summer and autumn, and a minimum in spring. In fact, Atlantic fronts can also pass in summer. In winter, precipitation usually occurs in the form of snow.
Here is the average precipitation.
Average precipitation - Stockholm
StockholmJanFebMarAprMayJunJulAugSepOctNovDecYear
StockholmJFMAMJJASONDY
Prec. (mm)453025303545607560505550560
Prec. (in)1.81.211.21.41.82.432.422.2222
Days1086778910991011104

Winter is cold: the average temperature drops below freezing (0 °C or 32 °F) from December to February, is around freezing in March, and typically exceeds the freezing point, with the melting of snow, in late March or early April. However, the weather varies greatly depending on period: when mild currents from the Atlantic Ocean prevail, the temperature can exceed freezing by a few degrees even in winter, and rain can fall instead of snow. In other periods, the temperature sinks below freezing, down to -20 °C (-4 °F) and more, when an anticyclone of polar or Siberian origin moves over the city.

In the short spring, in April and May, the weather is variable, and there can be the first mild days, but also the return of cold weather. It can still snow in late April and it can still get cold in May, but at other times it can already get warm, sometimes in late April, but more often in the second half of May, when an anticyclone can move over Scandinavia, bringing a few early summery days.

Summer goes from June to August, and is a mild season, with highs around 20/22 °C (68/72 °F), and very long days. June is a bit cooler than July, but it has the longest days of the year. Hot temperatures are recorded rarely: occasionally the temperature can reach 28/32 °C (82/90 °F), but nice days can occur more frequently. However, sometimes at night it can be very cool, or even cold, since the temperature can drop below 10 °C (50 °F) even in this season. Summer at high latitudes begins to decline soon enough, so that in the second half of August the rains become more frequent, and the temperature begins to decrease.

Autumn, from September to November, is cool and quite rainy as early as in September, and then gradually becomes more cold and gray; also, the days shorten rapidly. In November, the temperature is just above freezing, and in this month typically the first snowfalls occur.

The amount of sunshine in Stockholm is good enough in May and June, and is still acceptable in July and August, while it is definitely low from November to January, when the sun is rarely seen. Given the northern latitude, the days are very short from November to January, so that in February, which is cold as January and can sometimes be even colder, the city becomes a bit brighter.

Stockholm

The sea is Stockholm is cold all year round: it reaches 15 °C (59 °F) in July, and 16 °C (61 °F) in August.
Here are the average sea temperatures.
Sea temperature - Stockholm
StockholmJanFebMarAprMayJunJulAugSepOctNovDec
StockholmJFMAMJJASOND
Sea (°C)3222510151613964
Sea (°F)373636364150596155484339

When to go

The best time to visit Stockholm is summer, from June to August: it's generally a pleasant period, although sometimes it can rain. The warmest month is July, but June is usually the sunniest month, and has the longest days (since the summer solstice is the 21st of June). However, it's better to bring some warm clothes also in summer, like a jacket or sweater for the evening or for any cool days.