Agrahayana or Margashirsha,^[1] (Sanskrit: आग्रहायण, romanized: Agrahāyaṇa, Sanskrit: मार्गशिर्ष, romanized: Mārgaśirṣa) is the ninth month of the Hindu calendar. In India's national civil calendar, Agrahayana is also the ninth month of the year, beginning on 21 November and ending on 20 December. Margashirsha means related to the Mrigashīrsha nakṣatra (asterism), which has been known since Vedic times. In Tamil, Margashirsha is also known as Margaḻi.

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In lunar religious calendars, Agrahayana/Margashirsha may begin on either the new moon or the full moon around the same time of year and is usually the ninth month of the year.

In solar religious calendars, Agrahayana/Margaḻi begins with the Sun's entry into Sagittarius and is the ninth month of the year.

Vaikunta Ekadashi, the Ekadashi (i.e. 11th lunar day) of Margashirsha month, is celebrated also as Mokshada Ekadashi. The 10th Canto, 22nd Chapter of Bhagavata Purana mentions the young marriageable daughters (gopis) of the cowherd men of Gokula worshiping the goddess Katyayani and taking a vrata, or vow, during the entire month of Margashirsha, the first month of the winter season (Śiśira), to get the god Krishna as their husband.^[2]

Bhairava Ashtami falls on Krishna paksha Ashtami of this month of Margashirsha. On this day, it is said that the god Shiva appeared on earth in the fierce manifestation as Bhairava. This day is commemorated with special prayers and rituals.

In Odisha, all Thursdays in this month are celebrated as Manabasa Gurubara, wherein Lady Lakshmi is worshipped by Hindu women.

In Tamil Nadu, during this month of "Margaḻi", women make "kolams" or "rangoli" early in the morning. Devotees usually go to temples early in the morning and recite Thiruppavai by Andal and Thiruvempavai by Manikkavacakar.^[3]

• Astronomical basis of the Hindu calendar
• Hindu units of measurement
• Hindu astronomy
• Jyotiṣa
• Precession of the earth, Equinox