The six Ruthus
| Ruthu | Tamil months | 2025 Start |
|---|---|---|
| Ilavenil (spring) Vasantha Ruthu | Chithirai, Vaikasi | 14 April 2025, 3:28 AM |
| Muthuvenil (summer) Grishma Ruthu | Aani, Aadi | 15 June 2025, 6:51 AM |
| Kaar (rainy season) Varsha Ruthu | Avani, Purattasi | 17 August 2025, 1:59 AM |
| Koothir (autumn) Sarath Ruthu | Aippasi, Karthigai | 17 October 2025, 1:53 PM |
| Munpani (early winter) Hemantha Ruthu | Margazhi, Thai | 16 December 2025, 4:26 AM |
| Pinpani (late winter) Sisira Ruthu | Maasi, Panguni | 12 February 2025, 10:02 PM |
Equinoxes and solstices
| Event | Time | Day length |
|---|---|---|
| March equinox | 20 March 2025, 2:31 PM | 12:07 hours |
| June solstice | 21 June 2025, 8:12 AM | 12:38 hours |
| September equinox | 22 September 2025, 11:49 PM | 12:07 hours |
| December solstice | 21 December 2025, 8:33 PM | 11:38 hours |
Ayanam
- Uttarayanam
- 14 January 2025, 9:02 AM
- Dakshinayanam
- 16 July 2025, 5:39 PM
How it is calculated
Each Ruthu is two Tamil months. A new Ruthu begins when the Sun enters Mesham, Mithunam, Simmam, Thulam, Dhanusu or Kumbam.
At an equinox, day and night are nearly equal. At the June solstice the day is longest in India; in December it is shortest. These are astronomical (Sayana) figures.
In the panchangam, Uttarayanam begins on Thai 1 and Dakshinayanam begins on Aadi 1.