A mild climate
Let's start by explaining that Switzerland is not covered in snow all year long, not even in winter. The higher areas, from about 1,500 m, are mostly covered in snow in winter time (late December to March). Mountains tops (3,000 - 3,500 m and up) are usually covered in snow all year long, although the amoun of white peaks in summer decreases due to global warming. The lower valleys have quite a mild climate though. The climate in Switzerland is strongly influenced by the relatively nearby Atlantic. Winds from the Atlantic transport moist and mild maritime air into Switzerland.
A varying climate
The mountains add an extra dimension to the "mild" Swiss climate, as the elevation is an important factor for the temperature at a specific location. Also, mountains can stop clouds, resulting in a moist cool climate on one side of a mountain, and a dry sunny climate on the other side.
And then there's the Föhn: a high speed cross-alpine wind, similar to the Chinook in the Rocky Mountains. If the wind blows from the South, it results in rain in the Southern Alps, and dry warm fall winds in the Northern Alps, causing the snow to melt quicker if it happens in winter or spring. If the wind blows from the North (more rare), it results in rain in the Northern Alps, and dry relatively cool high speed fall winds in the Southern Alps. The Föhn can occur all year long but especially in winter.
Because of all this, the weather for any specific place in Switzerland, especially in the Alps, is quite hard to predict. Temperatures up to 35 °C (95 F) are no exception in July and August. Temperatures of -15°C (5 F) are no exception in winter. You'd best be prepared for anything when packing your cloths.
Unfortunately, global warming has added an extra dimension to the climate in Switzerland.