How many days are in the season of winter?

Winter has one quarter of the days of the year.

Autum has another quarter of the days.

Spring also has a quarter, as does summer.

Each season has the same number of days. Of course, since the year has 365 days that works out to 91.25 days per season. Each season starts and ends at either a solstice or an equinox. (this is the usually definition... some places put it a few weeks earlier, but lets just deal with the majority of the world)

Here it is for the northern half of the world:

Spring Equinox, Start of Spring, March 20 or 21. Day & Night are equal

length. Days are getting longer, nights shorter.

Summer Solstice. Start of Summer, June 20 or 21. Longest Day of the year. Days start getting shorter, nights longer.

Fall Equinox. Start of Fall/Autum, Sept 22 or 23. Day and Night are equal length. Days still getting shorter, nights longer.

Winter Solstice. Start of Winter. Dec 21 or 22. Shortest Day of the year. Days start getting longer, nights start getting shorter.