Saint Sylvester, or New Year's Eve is commemorated twice: 31 December (cf. the Gregorian calendar) and 13 January (cf. the Julian calendar). The Silvesterkläuse put on their strange costumes and, ringing huge bells and singing a very slow yodel, walk in small groups from house to house, wishing people a happy new year.
Find out more here: www.myswitzerland.com/en-ch/silvesterklaeuse-in-urnaesch