Vancouver, where it's located

In Vancouver, the largest city of British Columbia, the climate is oceanic, cool and humid, with relatively mild, rainy winters and cool, fairly sunny summers. The west coast is the only part of Canada in which the average temperature remains above freezing (0 °C or 32 °F) even in winter, and Vancouver is the only major city not having a freezing winter.
Here are the average temperatures.
Vancouver - Average temperatures
Min (°C)1236912141411731
Max (°C)78101317202222191496
Min (°F)343637434854575752453734
Max (°F)454650556368727266574843

Precipitation is quite abundant, in fact, it amounts to 1,200 millimeters (47 inches) per year, but in the northern suburbs, closer to the mountains, it reaches 1,600 mm (63 in). It is more frequent in the period from November to March; summer is the driest season. Vancouver is not exactly on the ocean coast, since it is protected by the island of the same name, and overlooks the Strait of Georgia. In the west of the island, on the coast of the Pacific Ocean, it rains much more, since precipitation exceeds 2,500 mm (98 in) per year.
Here is the average precipitation in Vancouver.
Vancouver - Average precipitation

Winter, from December to February, is quite cold, in fact, the maximum temperature ranges between 6 °C and 8 °C (43 °F and 46 °F), but compared to the rest of Canada, it's much milder. The temperatures are similar to those of cities such as London and Paris.
The rains are frequent, and during the periods when mild and moist Pacific currents blow, the temperature can exceed 10 °C (50 °F). On the other hand, in periods of calm weather, fog can form.
On average, 38 centimeters (15 in) of snow fall per year, distributed in 11 days; however, snow usually does not last long because after a while, the mild currents from the ocean start to blow again. Sometimes, colder periods may occur, with frosts of a certain intensity. The temperature dropped to -13 °C (9 °F) in December 2008 and to -9 °C (16 °F) in February 2014.
North of Vancouver, the Coast Mountains, which culminate in Mount Waddington, 4,019 meters (13,186 feet) high, are very snowy and therefore host glaciers and permanent snow already at an altitude of less than 2,000 meters (6,500 feet); in fact, the landscape of the area, consisting of fjords and islands, has been shaped through millennia by glaciers.
In winter, you can go skiing in the mountains just outside the city, like Grouse Mountain and Mount Seymour, or in the more northern Coast Mountains; in 2010, Vancouver hosted the Winter Olympic Games.

Spring, from March to May, is initially very cool, if not cold, as well as rainy, but it gradually becomes milder, although a bit of cold at night is possible still in May. Also, sunny days become gradually more frequent.

Summer, from June to August, is cool, but also quite sunny: the maximum temperatures hover between 20 °C and 23 °C (68 °F and 73 °F), but the Pacific high-pressure system provides longer periods of good weather than in the rest of Canada. If in winter, Vancouver is the mildest large city of Canada, in summer, it is the coolest, but also the sunniest. On the west coast of Vancouver Island, summer is even cooler: in Tofino, the average maximum temperature in July and August is 18/19 °C (64/66 °F).
Unlike in the other seasons, in summer, there can be a considerable number of consecutive days with good weather, however, there can be rainy periods even in this season, with cloudy skies, rain and cool temperatures, with highs remaining around 15/18 °C (59/64 °F).
Hot days, with highs above 30 °C (86 °F), are rare, and sometimes, they do not occur for an entire summer.

Autumn, from September to November, is initially mild and pleasant, while by October, it becomes gradually colder and definitely rainy. November is a very rainy month.

Vancouver from above

The amount of sunshine in Vancouver is scarce in late autumn and winter, when the sky is usually cloudy, while in summer, it becomes good, since there are many sunny days. Here are the average sunshine hours per day.
Vancouver - Sunshine

The sea in Vancouver and along the coast of British Columbia is cold even in summer, as you can see in the following table.
Vancouver - Sea temperature
Temp (°C)8881012131414131198
Temp (°F)464646505455575755524846

Best Time

The best period to visit Vancouver goes from mid-May to late September, with a preference for July and August, since they are the warmest and driest months.