The Keralites follow a Hindu sidereal calendar called the Kollavarsham. Initially the Malayalis followed the Kali abda based on the Hindu epic Kalki Purana. According to this there are 4 yugas or eons- Dwapara Yuga, Treta Yuga, Satya Yuga and the Kali Yuga. After the Kali Yuga, all of creation would be annihilation and new Sristi (creation) would be brought into existence again, thus heralding Satya Yuga. But in honor of the Seer Shankaracharya, a new calendar was adopted in Kerala called the Kollavarsham or the Malayalam Calendar. The Malayalam Era (ME) commenced in 825 AD. The word Kollavarsham has been derived from the Malayalam words, 'kollam and 'varsham'.

There are many conflicting views about the origin of Kollavarsham in 825 AD. Some believe that it is the calendar starting with the settlement of the Nestorian merchants in Koruke Ni Kollam, near Kollam. Others believe it was the year of the completion of a Shiva temple built by King Udaya Marthanda Varma. 825 AD denotes the year Saint Shankaracharya attained Samadhi (freedom from his worldly body). This date is ascertained with reference to Kali Dina Samkhya "Aachaarya Vaagabhedyaa" as mentioned by "Paralper" or "Katapayaadi". Others believe that the Malayalam Era started in 825 AD because that was the year when Shankaracharya carried out the 64 Anaachaarams which are a list of do's and don’ts and special customs and conventions for Namboodiris. However, since it is mentioned in the Mampalli plates of SriVallabhan Kotha in the year 973 AD (149 ME), we may assume that the Malayalam Calendar (Kollavarsham) had been widely accepted by then.

It is a solar sidereal calendar meaning that the entire year is the time the earth takes to complete a single orbit of the sun. Like the Roman calendar the Malayalam Calendar also is based on 12 months. But these months are named after the star constellations rising on the orbit of the earth. The year begins on Onam day, sometime in late August-early September. This post harvest festival is celebrated with great enthusiasm and cheer in Kerala. The 12 months of the Malayalam Calendar (Kollavaesham) are:

Chingam (Leo)	August-September
Kanni (Virgo)	September-October
Thulam (Libra)	October- November
Vrishchikam (Scorpio)	November- December
Dhanu (Sagittarius)	December- January
Makaram (Capricorn)	January- February
Kumbham (Aquarius)	February- March
Meenam (Pieces)	March- April
Medam (Aries)	April- May
Edavam (Taurus)	May- June
Midhunam (Gemini)	June- July
Karkidakam (Cancer)	July- August

The Malayalis calculate their festival days and the other occasions (such as rituals connected with the forefathers) and even agricultural activities based on the Kollavarsham. They call the 7 days of the week, starting Sunday, Njayar, Thinkal, Chouwa, Budhan, Vyazham, Velli and Sani respectively.