You'll see a broad range no matter when you go. We went last Sep and it was fairly warm in Lugano but we walked around in snow at Jungfraujoch. I'd worry more about crowds and would avoid Jul or Aug - which is why those tour dates tend to fill last. One nice thing about this tour is you don't need much in the way of dressy clothes. More important to pack a wardrobe that lets you layer and adjust as needed. Also comfortable walking shoes.
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You'll see a broad range no matter when you go. We went last Sep and it was fairly warm in Lugano but we walked around in snow at Jungfraujoch. I'd worry more about crowds and would avoid Jul or Aug - which is why those tour dates tend to fill last. One nice thing about this tour is you don't need much in the way of dressy clothes. More important to pack a wardrobe that lets you layer and adjust as needed. Also comfortable walking shoes.