The Hindu Lunar Calendar Explained: Tithi, Paksha and the Months

The daily Panchang tells you about today. Behind it sits a beautiful structure, the Hindu lunar calendar, which decides when festivals and vrats actually fall. Understanding this structure is not just academic knowledge. It is the key to knowing why Diwali lands on a different date every year, why your grandmother celebrates her birthday on a tithi rather than a fixed number, and why two devout families in the same city might observe Ekadashi one day apart. Here is how the whole system is built, and why it is far more precise than it first appears.

The Tithi: A Lunar Day

A tithi is the time the Moon takes to move exactly 12 degrees ahead of the Sun in the sky. Because there are 360 degrees in the zodiac, you get exactly 30 tithis in a complete lunar month. This is where the elegance begins, but also where the complexity creeps in.

The Moon does not travel at a perfectly uniform speed. Its orbit is elliptical, so it moves faster when it is closer to Earth and slower when it is farther away. This means a single tithi can last anywhere from about 19 hours to just over 26 hours. A tithi does not begin at midnight or at sunrise. It begins the moment the Moon completes that 12-degree separation from the Sun, and it ends the moment the next 12-degree gap is complete. A tithi can therefore start at 3 in the afternoon and end at 8 the next morning.

This variability gives rise to two important phenomena. A kshaya tithi is a tithi that is skipped entirely because it begins and ends within a single sunrise-to-sunrise window, never touching a sunrise at all. A vriddhi tithi (also called adhik tithi) is one that spans two sunrises and is therefore counted on two consecutive days. Both situations require careful calculation, which is exactly why reading a printed calendar from a different city can mislead you about the correct day to observe a fast.

The Two Pakshas

  • Shukla Paksha: the bright fortnight, running from the new moon to the full moon, as the Moon waxes and grows more visible each night.
  • Krishna Paksha: the dark fortnight, running from the full moon back to the new moon, as the Moon wanes and the nights darken.

Each paksha contains 15 tithis, numbered Pratipada (1) through Chaturdashi (14), ending in Purnima (the full moon, tithi 15 of Shukla Paksha) and Amavasya (the new moon, tithi 15 of Krishna Paksha). The paksha system is so deeply woven into Hindu life that many people still introduce auspicious events by reciting the paksha and tithi before the date.

The Lunar Months

Twelve lunar months form the Hindu year. The months, in order, are Chaitra, Vaishakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartik, Margashirsha, Paush, Magha and Phalguna. Each month is named after the nakshatra, the lunar mansion, near which the full moon falls during that month. When the full moon is near Chitra nakshatra, the month is Chaitra. When it is near Vishakha, the month is Vaishakha, and so on.

Two distinct conventions exist for when a month officially starts and ends. The Amanta system, widely used in Maharashtra, Gujarat, Karnataka, Andhra Pradesh and Tamil Nadu, treats the new moon (Amavasya) as the end of one month and the beginning of the next. The Purnimanta system, common in North India including Uttar Pradesh, Punjab and Himachal Pradesh, treats the full moon (Purnima) as the month's closing point. Both systems count the same tithis, but the boundaries of the month are drawn differently. This is why a family in Delhi and a family in Mumbai might refer to the same day by different lunar month names.

Adhik Maas: The Leap Month

The lunar year, made up of 12 lunar months, adds up to roughly 354 days, about 11 days shorter than the solar year of 365 days. Left uncorrected, this gap would push seasonal festivals like Holi and the harvest celebrations steadily backward through the solar calendar, eventually landing them in completely the wrong season. To prevent this, the Hindu calendar inserts an extra month called Adhik Maas (also known as Mal Maas or Purushottam Maas) roughly once every three years.

The rule for inserting Adhik Maas is astronomical and elegant. In a normal lunar month, the Sun moves from one zodiac sign into the next, a transit called Sankranti. In Adhik Maas, the Sun does not change zodiac sign at all during the entire lunar month. That month is therefore declared the extra, or intercalary, month. It takes the name of the month that follows it. So if the month before Vaishakha has no Sankranti, that extra month is called Adhik Vaishakha. Religious activities like marriages and other major samskaras are generally avoided during Adhik Maas, though it is considered highly auspicious for devotional practices and charity.

A Practical Example: Finding Ekadashi

Consider someone in Chennai wanting to observe Ekadashi, the 11th tithi, which is one of the most widely kept fasts in Hinduism. Ekadashi falls twice every lunar month, once in Shukla Paksha and once in Krishna Paksha. But because tithis float against the clock, the 11th tithi might begin at 2 a.m. one month and at 6 p.m. the next. The general rule is that Ekadashi is observed on the calendar day during which the 11th tithi is present at sunrise. If the 11th tithi begins after sunrise on Tuesday and ends before sunrise on Wednesday, it touches neither sunrise cleanly, creating a rare Kshaya Ekadashi scenario with its own specific rules. Someone in Kolkata observing the same Ekadashi might find that the sunrise-tithi alignment falls one day differently than it does in Chennai. This is precisely why location-specific calculation matters so much.

Why This Matters for Festivals

Because festivals are fixed to tithis and not calendar dates, their position in the Gregorian calendar shifts every year and can even differ by city or time zone. Diwali is always Amavasya of Kartik month, but the clock time of that Amavasya changes. Navratri begins on Chaitra Shukla Pratipada, but which solar date that corresponds to depends on where you are and when the Moon reaches that precise degree. The free CosmosPandit app computes accurate festival and vrat dates from real astronomical positions for your exact location, so you never have to guess. You can also use the CosmosPandit compare tool to see how the same tithi lands differently across two cities, which is especially useful for joint family celebrations spread across the country.

Frequently Asked Questions

Why does the same festival sometimes fall on different days in different Indian states?

The difference comes down to two factors working together. First, the Amanta and Purnimanta month systems draw the boundaries of a lunar month at different points, so people in North India and South India may technically be in different lunar months on the same day. Second, because a tithi is defined by a precise degree-gap between the Moon and Sun, the tithi active at sunrise depends on your geographic longitude and latitude. A city in the east experiences sunrise earlier than a city in the west, so the tithi ruling that sunrise can be different. This is why a location-aware tool like the CosmosPandit app gives you far more reliable festival dates than a generic printed almanac.

What is the significance of Amavasya and Purnima in daily Hindu life?

Amavasya, the new moon, is traditionally associated with ancestor rituals called Pitru Tarpan. It is considered an especially powerful time to offer water and prayers to departed ancestors, and many people observe fasts or visit sacred rivers on this day. Purnima, the full moon, carries a different energy. It is linked to abundance, heightened spiritual receptivity and the peak power of deities like Chandra (the Moon god) and Satyanarayan. Many temples hold special pujas on Purnima, and several major festivals including Guru Purnima, Sharad Purnima and Buddha Purnima are anchored to specific full moons. Both tithis are considered highly significant for meditation, prayer and charity.

Is Adhik Maas considered inauspicious, and what should be avoided during it?

Adhik Maas has a nuanced reputation. It is generally considered unsuitable for major life events like weddings, sacred thread ceremonies (Upanayana), griha pravesh (entering a new home) and other one-time samskaras that carry lasting consequences. This is because Adhik Maas is seen as a period outside the regular solar-lunar rhythm, and important transitions are preferred at conventionally auspicious solar junctions. However, the month is simultaneously regarded as deeply auspicious for personal devotion. Continuous chanting, reading of scriptures, fasting, charitable giving and worship of Lord Vishnu (after whom it is also called Purushottam Maas) are all highly recommended. Think of it less as an unlucky month and more as a sacred pause dedicated to inner spiritual work rather than outward worldly milestones.