Another factor
(although it may have been a very small one) in the
relocation of Christmas is that (in the
days of yore), September was
harvest time, and there was much work to be done to prepare for the
winter. By the time late December rolls around, all the crops have been dealt with and all that's left to do is sit around
shivering and hoping you
survive the winter.
Having the massive
festival in
December rather than
September therefor serves a double purpose: it prevents it from interfering with anything
important, and it gives the
peasantry something to cheer them up and help them get through the
horrible, deadly cold.