The Kerala New Year (Malayalam Calendar) is on the day of vernal equinox, i.e., the day when the sun is considered to move from the southern to the northern hemisphere. The day falls in the first Malayalam month of Chingham ചിങ്ങം according to the Kollam Era. Till the modern Malayalam calendar Kollam Era was adopted in 825 CE 1st day of the Malayalam month of Medam മേടം was considered to be the new year. This day is celebrated in Kerala as Vishu today and it coincides with similar new year festivities in several other parts of India like Tamil New Year in Tamil Nadu.
References
Kerala New Year Wikipedia(Text) CC BY-SA