January/February: winter high season
These are real winter months. It's cold and days are short (day light from about 8 AM to 5:30 PM). Snow is practically guaranteed from elevations of 1500 m. There can also be snow in the valleys and larger cities.
This is a good time for winter sports and cosy city trips. Hiking is only possible in lower elevations or on prepared winter trails.
March: early spring
March is when the first signs of spring can be found in warmer regions like Lake Geneva and the Ticino. It's pretty chilly in most regions though. In the mountains it's still winter with lots of snow.
High ski resorts will be operational. Lower ones may close around this time.
April/May: low season
Most ski resorts will be closed by the end of April. Spring is coming, especially in the lowlands and around the lakes.
More and more hiking trails will be accessible. By late April/early May, trails up to 2000 m should mostly be clear of snow, but can still be muddy. This is not a guarantee and varies each year.
The months between winter and summer are pretty quiet: some hotels are closed, as well as cable cars. There is still plenty to do and see, but you need to prepare and check timetables beforehand. Scenic train rides, for example, can be done all year round, and this is actually a good time: you can watch the remains of winter as well as green valleys.
June: early summer
Summer has started. There's day light from about 5:30 AM to 9:30 PM, and the weather is mild or warm.
This is a wonderful time for hiking. The highest trails may still be inaccessible because of snow, but the majority will be open. Cable cars and cogwheel trains that were closed in low season will be operational again. By late June, all pass roads that were closed for winter should be open. Opening dates vary per season and per pass.
July/August: summer high season
Although the weather varies by the day throughout the year, it's usually pretty warm in July and August. Average temperatures are rising because of global warming. Some days can actually be too hot for some activities, like city trips.
This is the time for outdoor activities: hiking, biking, paragliding and other sports, enjoying the views from the peaks. It's a perfect time for hiking in high terrain. By mid July, these trails should be clear of snow. Summer has traditionally been popular among tourists in Switzerland, just like winter.
September: perfect for outdoor activities
A wonderful month which is still summery, but usually not as hot as July and August. By the end of the month one can enjoy the first signs of fall when leaves start to color.
September is considered the best month for high altitude hiking. It's also very suitable for other outdoor activities.
October: low season
The weather can still be pretty mild in October, although this is fall and it might as well be chilly. The first snow fall will occur in the mountains. Days are getting shorter, with sunrise around 7:45 AM and sunset around 6:30 PM.
There are still lots of opportunities to hike, but higher trails may get snow covered, especially in the second half of the month. Cable cars and cogwheel trains may either close around this time, or not operate for a week or two because of maintenance. Pass roads start to close for winter in October.
November: low season and chilly
Things are getting chilly in November, although anything is possible, ranging from mild days to snow. The first ski slopes of high winter sport resorts like Zermatt and Verbier may open, but this is definitely not guaranteed.
Many lower hiking trails will still be open, but one has to carefully check for the conditions. It's low season: cable cars and cogwheel trains may close for maintenance, so planning ahead is essential.
December: Christmas atmosphere
December marks the transition to winter. The first half of the month is similar to November. Winter should set in during the second half. However, snowfall tends to start later in the season due to global warming. A white Christmas is not a guarantee, although still common in the high mountains. Don't expect snow in the valleys and large cities like Zurich. It's possible but uncommon.
It's still low season until mid-December. When Christmas approaches, things get lively and winter sports resorts open up. The two weeks around Christmas and New Year are the most popular weeks of the year among tourists.
December is a good time to visit Christmas markets, and for city trips to Zurich, Lucerne and Bern for example. Days are really short with sunrise around 8 AM and sunset around 4:45 PM.