tanith, I don,t regard December as midwinter except as concerns daylight. Temperature-wise February is nearly always the coldest month where I live.
Given that August (and not June/"midsummer") is usually the hottest month in the northern hemisphere, it's not really surprising that February should be the coldest month, after a long cooling down period, just as August tends to be the hottest (though not necessarily sunniest) after a long warming up period.
December has certainly been mild (for the most part, though we've had cold snaps) but it has been colder than October and November here.