The Met Office official "winter" is December, January and February. To
me, this makes a lot of sense. In just over 2 weeks time the evenings
will start to get lighter again, although the mornings get darker still
for a few more weeks. Once January has passed the nights are quite a bit
lighter. By mid-February there are usually clear signs that spring is
on its way. Mind you, snow is not unusual in January and February and
the summer bird migrants don't usually arrive until April mainly.