Old English, "fourth season of the year," from Proto-Germanic *wentruz (cf. Old Frisian, Dutch winter, Old Saxon, Old High German wintar, German winter, Danish and Swedish vinter, Gothic wintrus, Old Norse vetr "winter"), possibly from PIE *wed-/*wod-/*ud- "wet" (see water), or from *wind- "white" (cf. Celtic vindo- "white").
The Anglo-Saxons counted years in "winters," cf. Old English ænetre "one-year-old." Old Norse Vetrardag, first day of winter, was the Saturday that fell between Oct. 10 and 16.