This festival is currently celebrated on 14 or 15 January but due to axial precession of the earth it will continue to shift away from the actual season. The season occurs based on tropical sun (without ayanamsha). The earth revolves around sun with a tilt of 23.45 degrees. When the tilt is facing the sun we get summer and when the tilt is away from the sun we get winter. That is the reason when there is summer north of the equator, it will be winter south of the equator. Because of this tilt it appears that the sun travels north and south of the equator. This motion of the sun going from south to north is called Uttarayana – the sun is moving towards north and when it reaches north it starts moving south and it is called Dakshinayana – the sun is moving towards south. This causes seasons which are dependent on equinoxes and solstices.