Dhanu Sankranti marks the Sun's passage into Dhanu, the ninth sign of the sidereal zodiac, corresponding to Sagittarius. It is determined by the Sun reaching a longitude of two hundred and forty degrees against the fixed stars, using the Lahiri ayanamsa, and normally falls in mid-December.
As with every sankranti this is a solar event, defined by the Sun's ecliptic longitude and independent of the lunar months that carry festivals like Diwali. The transit instant is converted to local clock time, so the observed date and the timing of associated bathing or giving can differ by a day between cities.
The solar month that begins here, called Dhanurmasa or Margazhi in the Tamil calendar, is regarded as a time for devotion rather than new ventures, and many communities avoid marriages during it. In Odisha the day opens the Pahili Bhoga offerings at the Jagannath temple, and in Tamil Nadu it begins the early-morning Margazhi hymn-singing.