The Winter Solstice (or Midwinter's Day) is the polar opposite of the Summer Solstice, and an event appearing in Animal Crossing: City Folk and Animal Crossing: New Leaf
The player talking to Tortimer on Midwinter's Day
The Winter Solstice appears as Midwinter's Day in European copies of Animal Crossing: City Folk. Tortimer will stand outside the Town Hall. If the player speaks to him, they will obtain a Snow Globe.
This event is celebrated by Isabelle standing out in the Event Plaza, and she will give the player a Blue Glow Stick, which cannot be obtained anywhere else. At Winter Solstice, it will be night all day, and many villagers, especially snooty ones and Isabelle, will complain about the cold. A four-person cutout board will be stood out in the Event Plaza as well.
Real life definition
The winter solstice occurs when the tilt of a planet's semi-axis, in either the northern or the southern hemisphere, is most inclined away from the star (sun) that it orbits. This happens twice each year, at which times the sun reaches its lowest position in the sky as seen from the north or the south pole. However, what is seen in New Leaf is actually known in the real world as a "polar night", in which the sun is not visible for more than 24 hours, which occurs in the arctic or antarctic circle, during the days near the winter solstice.