Chamonix reaches 18.5°C today under broken cloud. By Sunday the high drops to around 11°C, with showers on Saturday and rain to follow. Zermatt stays dry until Sunday. That sets the order. Take the long ridge or hut walk on Friday or Saturday, and save Sunday for a shorter valley loop. Waterproof trousers belong in the pack from Saturday on.
Waterproof trousers ↗