Declension of Vánoce

It is funny how Vánoce, a plurale tantum meaning Christmas in Czech, has Vánocům in the dative, not *Vánocím, as you would expect if you were to compare it with the word noc, night, from which it is derived. Slovak is again more regular in this regard: noc - dative plural: nociam, Vianoce - dative plural: Vianociam.