Pearlywhites21
In case you were considering the Glacier Express, there is a note on their website about ‘Glacier Express not running’ during that period: https://www.glacierexpress.ch/en/timetable
You might want to contact them directly for more details (and report back here for others?)
Don’t worry, Zermatt will not be isolated from the rest of the world because of that. Regular trains will still operate between St. Moritz and Visp, and as Danhiel already stated above, some replacement bus service will be established beween Visp and Zermatt. Obviously that adapted bus timetable is currently being figured out and therefore not yet available.