If you were one of the Celts of Britain, your day would begin at sunset. (That's still true of, for example, the Jewish calendar.) Your year would be divided into the dark half and a light half. The year began at what the Welsh called Calan Gaeaf and the Irish called Samhain. This was around Halloween on the Gregorian calendar, and the start of the dark half of the year.
Six months later was Calan Haf (Bealtaine), on what's now the first of May, the beginning of the light half of the year.