Isn't it the first day of February on the Celtic calendar on the basis that the winter solstice is basically the middle of winter, so winter should be (roughly) equal lengths either side of the solstice?
The only other thing I can contribute was that on an episode of Q.I., they were asked what is it that travels north (in the UK) at a speed of eight miles per hour, and the answer, which they didn't get, was 'spring'.
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