Zermatt when to go?/ Zermatt when to ski?

It very much depends on what you are looking for. The obvious choices are Christmas, New Year, and Easter. If you want to celebrate the occasion surrounded by snow, then that could be ideal. However, if you want quiet slopes and excellent snow, my recommendation would be January and March. Though the temperatures are often quite cold in January, the conditions mid to late January are often fab! March has longer days and usually lots of sunshine. Perfect for some time on a sunny terrace.