Have you ever noticed how some Hindu festivals occur on different dates, each year? It may be around the same time period, say a month, but the dates tend to vary. Have you ever wondered why that happens? Or do you just celebrate it each year on the day your community celebrates, without questioning it?

Actually, the idea behind this is quite simple.

Most festivals in India are celebrated as a commemoration of a particular event in the Ithihasa or a person/God. For example, the celebration of Diwali, in the Northern part of India, is a commemoration of the return of Lord Rama to Ayodhya after his battle with Ravana, and the defeat of the demon Narakasura by Lord Krishna and Goddess Satyabhama, in the Southern part. Normally, these two dates occur one day after another, and sometimes, even on the same day.

Hindu festivals — How do we know when to celebrate them?

Although these two may be the most popular reasons why Diwali is celebrated, there are multiple other reasons as well, which vary depending on the geography and the cultural history of the place.

But how do these dates, come together almost every year? How do Janmashtami, Navaratri, Ganesh Chaturthi, and many other such festivals differ in their dates, each year, but occur around the same period of a year?

To understand this, it’s important to equip yourself with the concept of a Lunisolar calendar. It is known as the Panchāngam. A Solar Calendar is a representation of the time of the year with the Sun’s apparent position with reference to distant background stars. This calendar is what the Greek astrological Zodiac signs are based on. A Lunar Calendar is a representation of the time of the month, with reference to the phase of the Moon (New Moon to New Moon is 1 month). A Lunisolar Calendar is a combination of both such types and serves as a representation of the phase of the Moon and the time of the year concerning the Sun.

If you are familiar with the Hindu culture, you must have heard the words “Ashtami”, “Chaturthi”, “Ammavasai”, “Pournami” etc. The idea behind this is the Lunar cycle. Each phase of the moon is given a name, to indicate the time of the month.

Ammavshya/Ammavasai — New Moon

Shukla Paksha (Waxing Phase)

Then, the name of the day (depending on the increase in the phase of the moon) is given as —

Prathamai — 1st Day
Dvithiyai — 2nd Day
Thrithiyai — 3rd Day
Chaturthi — 4th Day
Panchami — 5th Day
Sashti — 6th Day
Sapthami — 7th Day
Ashtami — 8th Day
Navami — 9th Day
Dasami — 10th Day
Ekadasi — 11th Day
Dvaadasi — 12th Day
Thrayodasi — 13th Day
Chathurdasi — 14th Day

Poornima or Pournami — Full Moon. After which the waning phase or the Krishna Paksha begins which will follow the same set of names for the waning moons.

14 days of Shukla Paksha and 14 days of Krishna Paksha add up to 28 days. Then, adding the Full Moon and the New Moon itself, makes the Lunar cycle a 30-day period (30 is an approximated value).

The Solar Calendar followed in India is similar to the Gregorian Calendar, except the months begin on a different day, which is the 15th of every month(with reference to a Gregorian month).

Here is the list of months according to the Tamil and Sanskrit languages:

Chitthirai — Chaitra — Mid April to Mid May
Vaikāsi — Vaisākha — Mid May to Mid June
Āni — Jyastha — Mid June to Mid July
Ādi — Ashādha — Mid July to Mid August
Āvani — Shrāvana — Mid August to Mid September
Purattāsi — Bhādrapada — Mid September to Mid October
Aippasi — Ashwini — Mid October to Mid November
Kārthikai — Kārthika — Mid November to Mid December
Mārgazhi —Mārghasirsa — Mid December to Mid January
Tai — Pausha — Mid January to Mid February
Māsi — Māgha — Mid February to Mid March
Panguni — Phalguna — Mid March to Mid April

The references to certain events, which are today celebrated as Diwali, Janmashtami, Ganesh Chaturthi, etc. are mentioned with the month and the lunar day. And because these dates do not line up on the same day every year, they tend to vary.

Janmashtami — A celebration of the birth of Lord Krishna — Born in the month of Āvani/Shrāvana, on the 8th lunar day, Ashtami, of the waning phase (Krishna Paksha). Hence the name Janma (birth) + Ashtami.

Some communities celebrate these festivals on the day the moon rises and sets with the star (among the 28 nakshatras) in which the moon is said to have resided, on that day. For example, the birth star of Lord Krishna, that is the Rohini Nakshatra (Aldebaran in the constellation of Taurus) is where the moon is believed to have been located in. So, they celebrate it on the day the moon resides in the Rohini nakshatra in the month of Āvani/Shrāvana.

Similarly, all Indian festivals follow such a method of calculation of their date. And that’s why these dates tend to differ. Also, this is why we are able to commemorate an event/person from ancient India’s history that is believed to have occurred thousands of years ago, at a time when the concept of a western calendar did not exist, and still celebrate it on the same day, even in the 21st century.

Hindu festivals — How do we know when to celebrate them?