midwinter which is an astronomical feast of quite definite timing occurs in about the middle of December

Don't think so. You're assuming that each "season" is defined so that "mid-season" (e.g., midwinter) falls on the astronomical solstice or equinox. However, the definition of "season" is a period beginning at solstice or equinox (and ending at the next equinox or solstice; that is, at the start of the next season).
http://bartleby.com/61/51/S0195100.html: "Each season, beginning astronomically at an equinox or solstice, is characterized by ..."

Max, notice my care to avoid implying that December 23 is the winter solstice.