Mesha Sankranti marks the Sun's passage into Mesha, the first sign of the sidereal zodiac, corresponding to Aries. The instant is fixed by the Sun's longitude reaching zero degrees measured against the fixed stars, using the Lahiri ayanamsa, and it usually falls in mid-April.
This is a solar reckoning and is independent of the lunar months that govern festivals like Diwali or Holi: it does not depend on a tithi or on the phase of the Moon, only on where the Sun sits along the ecliptic. The observed date can move by a day between cities because the transit instant is converted to local time, and customary practice ties bathing and giving to the sunrise nearest the sankranti.
It opens the year in the solar calendars of several regions and is kept as Baisakhi in Punjab, Pohela Boishakh in Bengal, Puthandu in Tamil Nadu, Vishu in Kerala, Bihu in Assam and Maha Vishuba Sankranti in Odisha.