Thank you for your answer, Pocus!
So it's true that it is hard-coded as it is (via months/ 1/12s of a year) linked to a "year". And since the maximum number of turns per year available is 48, I have no chance to get winter to work on a different scale. I was thinking about using two years as one year (48x2=96 turns à 7 (half-)days) but it seems that this wouldn't work then as the winter-mechanic (and, not that much of a problem, the labeling of the current date/turn) would interfere. Tant pis, but thanks for clearing that up! Much appreciated!