Pougheapie55 The timetables in France are published later. The trains run all year, but the line may be interrupted for engineering works. In this case you have replacement buses.
I suggest to wait until the French timetable is published. Please note that no seat reservations are available for this relation and the trains from Martigny to Chamonix are local trains, second class only.
If you search a relation on sncf-connect.com from Vallorcine to Chamonix for this date, a window is displayed and you can give your email address to receive a notification when the tickets are available. The site sbb.ch will be adapted some days later. But don’t buy tickets from Zermatt on sncf-connect.com, the tickets are not available as PDF. It is better to buy tickets on sbb.ch.