The chapel is continually open, and is accessible by both public transport and car. (These are the directions that I found most helpful.) I would heavily recommend either taking the train or plane to Zurich, then renting a car to drive the 3.5 hours to Sumvitg. I was here in mid-September, so the leaves were starting to change - but I imagine it would be even more magical in the middle of a snow-blanketed winter.  A Pilgrimage to Zumthor's Chapel by Tiffany Chu