Hi sreejith2409 -
Adding to what Arno and Brahms said about the unpredictability of the weather, this link gives a good perspective for all of Switzerland:
The Alps are high enough and large enough to have a strong influence on the local weather.
May is a usually quite pleasant at lower elevations . The area near Luzern (at about 400 meters above sea level) will be warm and the farm land in the Emmental will be well into Spring. Crops will be growing in the fields. In some years, depending on crop rotation schedules, there will be large patches of bright yellow "Rap" (English "rape seed") in the fields- images attached. There will still be snow on the upper elevations higher mountains near Luzern. Although there will definitely be rainy and cloudy days, there will be a lot of sunny ones, as well. Since I like the parts of Switzerland that are at lower altitude, I often visit in May.
However, if I go to to the Jungfrau region in May, I'll find that there is snow on the high trails, which are no longer groomed for walking or snowshoeing or cross country skiing, as Spring is beginning. The ski season has ended, but there is too much snow at 2000+meters to use many of the hiking trails. Since there are neither skiers nor hikers, many hotels (but not all) are closed for vacations or maintenance. Many restaurants are closed. The cable car from Wengen to Männlichen typically begins its summer season sometime in early June depending on weather. It may run earlier on weekends, or later than early March if there is still skiable snow. The really spectacular easy hiking rail from Männlichen to Kleine Scheidegg ( pictures attached) usually opens in mid-June; sometimes earlier. Even in mid-June it may close on some days. It depends on how much snow is left in the shaded area near Tschuggen along the trail.
So, I wait till late June. High alpine trails are generally available in July, but the chances are better as the month progresses. August and September are good hiking months; August is the peak tourist season, since schools are closed and many companies close for vacation.
Certain tourist attractions are available year around. The cog rail train from Lauterbrunnen to Wengen to Kleime Scheidegg (where there is a branch line to Jungfraujoch) and down to Grindelwald runs every day. The series of cableways to Schilthorn from Stechelberg on the valley floor runs most of the year.
The timetable can help with specifics:
You may have bright clear days at any time of the year. The best chances are in the middle of Winter or July, August, and particularly September as well as most of October.