Lunar and solar years
Why do lunar festival dates change each year?
Twelve lunar months are about 11 days shorter than the solar year. See how that drift, and the extra month that corrects it, moves festival dates.
01 · Two calendars
Twelve lunar months are about 11 days shorter than the Sun’s year.
The Sun takes a year to pass through the twelve signs. Twelve new Moon to new Moon months take about 354 days, so each lunar year ends earlier against the solar one, until a month with no sign change is counted as an extra month.
02 · One festival, year by year
Diwali’s Amavasya slips back about 11 days, then jumps forward.
A festival tied to a tithi in a named lunar month follows that month. When the lunar year has twelve months the date falls earlier the next year; after a thirteen-month year it falls later.
Two clocks that do not divide evenly
A Panchang runs two calendars at once. The solar months follow the Sun through the twelve sidereal signs, one Sankranti to the next, and take a sidereal year of about 365.26 days. The lunar months run from one new Moon to the next, as the Moon’s month explains, and average 29.53 days. Twelve of them make about 354.4 days.
The gap is about 11 days a year. If nothing corrected it, a lunar month would drift through every season in about 33 years.
A month with no Sankranti
A lunar month takes its name from the sign change that happens inside it: the month in which the Sun enters Aries is Chaitra, the one in which it enters Taurus is Vaishakha, and so on. On average about once every 32 or 33 months the drift lets a whole lunar month pass with no Sankranti at all, because two new Moons fall inside the same sign. That month gets no name of its own. It is counted as Adhika (अधिक), an extra month, and takes the name of the month that follows it.
In 2026 the Sun entered Taurus at 05:58 IST on 15 May and Gemini at 12:28 on 15 June. The new Moons of 17 May and 15 June both fell between them, so the month from 17 May to 15 June 2026 was Adhika Jyeshtha. The year from one Mesha Sankranti to the next held thirteen lunar months.
What this does to festival dates
A festival kept on a tithi of a named month moves with that month. Diwali falls on the Amavasya that ends Ashwin, called Kartika Amavasya where months are counted from full Moon. That new Moon came on 21 October in 2025 and on 9 November in 2026. The years from 2024 to 2035 show the pattern: about 11 days earlier each year, then about 19 days later in a year with an extra month. Over the long run the festival stays in the same season, because the extra months keep the lunar year in step with the Sun.
The tithi and month fix only the window. The day a festival is kept depends on which part of the day the tithi must cover, so Diwali itself can fall on the day before the new Moon; why the same festival can have two dates explains those rules.
Further reading
- Why a fortnight can have two month names
- How an extra lunar month fits in
- How a lunar month name can be skipped
- Each month on its own: Chaitra, Vaishakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha, Pausha, Magha and Phalguna.
The lunar month guide lists the twelve names.
See today’s lunar month with the tithi on the Delhi Panchang, or explore another topic in Learn.