The Bengali Calendar, Explained

Every year the same question comes up in Chittaranjan Park: when is Pujo this time? It is a reasonable question, because the answer changes. Durga Puja is not in October the way Christmas is in December — it lands somewhere in a window that stretches from late September to late October, and the same drift applies to every Bengali festival the colony observes. This guide explains the machinery behind that, which is worth understanding once rather than re-asking each year.

Two clocks running at once

The Bengali calendar tracks both the sun and the moon, and that is the root of everything below.

The months are solar. Each begins when the sun enters a new zodiacal sign, which is why Poila Boishakh — the new year — sits reliably in mid-April and moves by at most a day. The Bengali era is counted as Bangabda, a numbering that runs several hundred years behind the Gregorian year.

The festivals are lunar. They are fixed to a tithi, a lunar day, within a named month. A lunar month runs about 29.5 days while a solar year runs about 365.25, so lunar dates slip backwards against the Gregorian calendar by roughly eleven days a year and are then pulled forward again when an extra lunar month is inserted to stop the drift compounding. The net effect is a festival that wanders inside a three- to four-week window.

The twelve months and six seasons

Month	Roughly	Season	Associated with
Boishakh	mid-Apr to mid-May	Grishmo (summer)	Poila Boishakh, Rabindra Jayanti
Jyaishtha	mid-May to mid-Jun	Grishmo	Jamai Shashthi
Ashar	mid-Jun to mid-Jul	Barsha (monsoon)	Rath Yatra
Shrabon	mid-Jul to mid-Aug	Barsha	Jhulan
Bhadro	mid-Aug to mid-Sep	Sharat (early autumn)	Janmashtami, Vishwakarma Puja
Ashwin	mid-Sep to mid-Oct	Sharat	Mahalaya, Durga Puja, Lakshmi Puja
Kartik	mid-Oct to mid-Nov	Hemanta (late autumn)	Kali Puja, Jagaddhatri Puja
Ogrohayon	mid-Nov to mid-Dec	Hemanta	Nabanna, the harvest
Poush	mid-Dec to mid-Jan	Sheet (winter)	Poush Sankranti, Poush Mela
Magh	mid-Jan to mid-Feb	Sheet	Saraswati Puja
Falgun	mid-Feb to mid-Mar	Bosonto (spring)	Dol Yatra
Choitro	mid-Mar to mid-Apr	Bosonto	Choitro Sankranti, Charak

Six seasons rather than four is not decorative. Sharat and Hemanta describe genuinely different parts of the autumn, and the split matters because the biggest festival of the year sits precisely at the junction: Durga Puja belongs to Sharat, and Kali Puja, a few weeks later, belongs to Hemanta.

How a tithi actually works

A tithi is the period in which the moon gains twelve degrees of angular distance on the sun. Thirty tithis make a lunar month, split into two paksha of fifteen each: shukla paksha, the bright fortnight from new moon to full, and krishna paksha, the dark fortnight back to new moon. The fifteenth tithi of the bright half is purnima, the full moon; the fifteenth of the dark half is amavasya, the new moon.

Because the moon’s speed varies, a tithi is not 24 hours. It runs somewhere between about 19 and 26 hours, and it can start and end at any time of day. Two consequences follow, and both explain things visitors find odd:

• A festival can look like it spans two dates. The tithi begins on one Gregorian day and ends on the next; which day the observance falls on depends on rules about when the tithi is current at the relevant time of day.
• Ritual windows are quoted to the minute. A midnight puja announced as running from 11:30 PM to 12:13 AM is not an arbitrary schedule. It is the span during which the required tithi is actually current. The temples guide lists the observances at the Kali Mandir that are timed this way.

Worked example: why Durga Puja moves

Durga Puja is observed on the sixth through tenth tithi of the shukla paksha of Ashwin. The names of the days are simply the tithi numbers — Shashthi (sixth), Saptami, Ashtami, Nabami, Dashami.

The festival is preceded by Mahalaya, the amavasya that closes the preceding dark fortnight and opens Devi Paksha, the fortnight of the goddess. That is a fixed lunar relationship: Mahalaya is always the new moon before, whatever the Gregorian date says.

Sandhi Puja is the sharpest illustration of tithi logic in practice. It is performed across the junction of Ashtami and Nabami — the last 24 minutes of one tithi and the first 24 minutes of the next — a 48-minute window that can land at any hour of the day or night. Puja committees plan an entire day around when that window falls. The Durga Puja guide covers what happens on each of the five days.

Why Kali Puja and Diwali coincide

Kali Puja falls on the amavasya of Kartik. Diwali’s main night, in the North Indian reckoning, falls on the same new moon. They are not two festivals that happen to collide; they are two traditions observing the same lunar moment in different ways, which is why Chittaranjan Park celebrates both on one night rather than choosing.

The same structure explains Kojagori Lakshmi Puja, which falls on the purnima of Ashwin — the full moon a few days after Durga Puja ends, and several weeks before Diwali. It is a distinct observance from the Lakshmi Puja performed during Diwali elsewhere in India.

The panjika, and why almanacs disagree

The calculations above are published in a panjika, the Bengali almanac. It is an annual volume giving the tithi for every day, the auspicious and inauspicious windows, the dates of every observance and the timings for rituals.

More than one panjika is in circulation, and they do not always agree, because they use different computational bases — one hewing to older traditional methods, another to corrected astronomical calculation. The differences are usually small and usually invisible, but occasionally two almanacs place a tithi on different days, and then two temples or two puja committees genuinely observe a ritual on different dates. Neither is wrong; each follows the almanac it has always followed. If you see a discrepancy between published dates for the same festival, this is nearly always the explanation.

What this does not tell you

This guide explains the mechanism, not the dates. For the actual dates observed in the colony, use the festival calendar, which lists them for the current year with venues and timings. Do not attempt to derive a date from the rules here — the calculation depends on precise astronomical positions, the observer’s location, and which almanac a particular temple follows, and an approximate answer is worse than no answer when someone is planning a trip around it.

Nor does it cover the civil Bengali calendar used for administrative purposes in Bangladesh, which was reformed to fix month lengths and does not track the traditional reckoning exactly.

The bottom line

Bengali months follow the sun and sit steady against the Gregorian calendar; Bengali festivals follow the moon and move. A tithi is a lunar day of variable length, a paksha is a fortnight, and a panjika is the almanac that turns the two into a date. Once that is clear, the pattern becomes predictable even if the dates are not: Saraswati Puja in Magh, Poila Boishakh in mid-April, Rath Yatra in Ashar, Durga Puja in the bright fortnight of Ashwin, Kali Puja on the new moon of Kartik. The question is never really which month — it is which tithi.