Gregor
The Best Month to Travel to Switzerland
Although Switzerland can (and should) be visited year round there is a time that stands out.
Switzerland has two distinct high seasons for tourism. There is winter (December-February) and summer (June-September). They are separated by shoulder seasons where the weather is changing often and many alpine destinations use this downtime for maintenance or a break, so the offering for activities and sights may be limited.

Switzerland has a moderate climate and is not enormously hot, cold, humid or dry. During the summer holiday months temperature are somewhere around 20-30°C (daytime), in the deep winter in January and February temperatures range from 0-10°C, sometimes even below zero. These temperatures are at low altitudes. Because Switzerland is not always the same altitude, temperatures may vary. In general one can assume that the temperature may decrease 1°C per 100m altitude gained. This means while on a hot summer day in Zurich with 30°C, at the top of Jungfrau, you will be met with temperatures 0°C and snow.
So, if you want the best of both worlds, then summer is your choice. But here we also have the temperature/ altitude situation mentioned before. While it can be already really warm in June, in the Alps it may still be fresh and not pleasant. The same goes for the end of summer, which generally is over in the Alps by the end of September. This leaves you a window of three months, out of which two fall in the high season for holidays.
This means September is the most "ideal" month to visit Switzerland, if you like to see all of what Switzerland has to offer and if you don't want it to be crowded.
The best months to visit Switzerland!

January
This is the best winter month in terms of crowds, as it is between Christmas and the ski holidays. It is also the coldest month and if you decide to spend the time at low altitude in the cities, then you will have to endure a lot of covered sky and it rarely snows there. So this is the time to go up to the Alps and enjoy the winter wonderland!
February
This winter at its finest because very often it's sunny everywhere, which means that it's not as freezing cold anymore. This is also the time a lot of tourists come for winter sports, so the ski resorts are full and prices high. But it is the nicest time to enjoy the snow in the Alps! Also here it can be very foggy in the cities.

March
In March, it's the time where the spring starts knocking on the door. It's not winter anymore, the snow starts to get slushy in the Alps and on some days you can skip the winter jacket. But this is a sandwich month of the shoulder season where neither the Alps or the cities are at their best side. The upside is that your trip will not cost as much during this time because of low demand!
April
In the cities at lower altitudes the trees and flowers start to come back to live and bloom. This is one of the nicest times for a city trip as also the locals tend to be happy after the sometimes very foggy winters. During April though the weather may change often. In the Alps, a lot of attractions and accommodations go on a break.

May
May is the month when it is getting pleasant at low altitudes and the nature is really colorful and pretty. In the Alps the majority of the snow has melted and spring also starts there, but many attractions are still closed. It is great to do a city trip during this time.
June
In June the Swiss summer starts. You can often wear summer clothes and it is pleasant everywhere. Also in the alpine locations summer is starting and you can go for hikes and start exploring the mountains as the majority of the snow has melted and the cable cars have opened up again. During this time not many visitors are here yet.

July
This is summer (when it's not raining for two weeks straight, which sometimes happens) in Switzerland! Enjoy everything you like, you can access the entire country, go for a swim, hike, bike, feel free and enjoy the long days! During this time the holidays start so it can be more crowded than in June. There are some outdoor festivals in all of Switzerland for you to enjoy.
August
Summer has had its peak in July, so it slowly gets more moderate. It is not as hot anymore but you can still do everything you planned and very often the weather is more constant than in July. In the Alps it super nice at this time, although there this is really the end of summer already. August 1 is our national day, so if you're a culture vulture then mark this day fat in your itinerary!

September
This may be your favorite month as the light just dips Switzerland in its best dress. It is so pretty everywhere and in higher altitudes the fall is starting already, meaning you get color overload! On top of that the crowds have left and you get to take it all in for yourself and move more freely. September is also the start of the hiking season for the Swiss, so this could be your favorite activity now.
October
Fall has arrived in Switzerland and the trees are in all different colors. This is a super time to do some hiking, whereas in the high Alps some mountains already have the first now. In the cities this time is about coming together on markets and celebrating food. Also it's a great time to enjoy a fondue as the temperatures start getting colder.
November
Many mountain resorts are in their shoulder season and life prepares for winter. It is often foggy and not the outdoors are not as pleasant anymore and many alpine attractions undergo maintenance during November. Now life moves more inside. If you want me to tell you which month I would skip, then it would be this one. But then again, there are good deals around.

December
Although it is not snowy everywhere, the lights, the Christmas markets and the festivity make life great during this month. Ski resorts open up usually in the middle of the month and life is good. If you want to spend time in the Alps, this is the time where it's most crowded and expensive as a lot of locals spend the Christmas holidays there. But you will be surprised how nice the cities are at this time with the locals being in a really good mood!