The World Season Calendar (WSC) is a proposal for calendar reform by author Isaac Asimov. In this calendar, the year is divided into four seasons of 13 weeks each. The seasons of the year are named A, B, C, and D; they correspond to the traditional fours astronomic seasons. December 21, which is on or near the time of the northern winter solstice on the Gregorian calendar, is called A-1 on the World Season Calendar. During leap year, an extra day called Leap Day is added at the end of season B. Leap Day, when it occurs, is day B-92, and is not assigned a day of the week.