The traditional East Asian calendars divide a year into 24 solar terms . Dàxuě or Taisetsu is the 21st solar term. It begins when the Sun reaches the celestial longitude of 255° and ends when it reaches the longitude of 270°. It more often refers in particular to the day when the Sun is exactly at the celestial longitude of 255°. In the Gregorian calendar, it usually begins around December 7 and ends around December 21 .