Bihu is the most important non-religious festival of the Assam and is observed by all irrespective of class and caste. There are three Bihu’s. In April it is Bohag Bihu , in October/November it is Kati Bihu and in January it is Magh Bihu. The Bihu festival celebrates the three seasons of spring, important to the largely pastoral people of Assam.

Bohag Bihu, the most important Bihu of all the three, is celebrated in the month of Bohaag (middle of April), the first month of the Assamese calendar and thus marks the advent of the Assamese New Year.

The Bohaag Bihu ushers in the period of greatest enjoyment and the spring season. Gay spirit of spring marks the celebration of Bohaag Bihu, which continues for three days. It marks the advent of seeding time, the "Kaati Bihu" marks the completion of sowing and transplanting of paddies, and the "Maagh Bihu marks the end of the harvesting period.

Bohaag Bihu is also called the "Rongali Bihu" or the Festival of Merriment. Rong in Assamese means joy.  As merriment  predominates the celebrations. Magh Bihu is also called "Bhogali Bihu" or the Festival of Food. The Kaati Bihu is also known as "Kongaali Bihu" or the Festival of the Poor.

On the eve of the Bihu the womenfolk clean the clothes and prepare special Bihu delicacies like 'Chira', Pitha, etc. In the rural areas the men folk remain busy in collecting necessary items such as 'Tara Pogha' (ropes for the cattle) prepared out of slices of 'Tara' - (an indigenous creeper) and vegetables such as raw turmeric, brinjal, gourd etc for the next days 'Garu Bihu'.

The first day of the Bihu is dedicated to the cattle (Goru), as cows and bullocks provide them with means of livelihood. On this day, early in the morning the cows and bullocks are ceremonially bathed in a river and ponds. Their feet are washed; horns and hoofs are painted various colours and are adorned with flower garlands. Cut pieces of the vegetables like brinjal, gourd etc. are offered to them. Their old ropes are cut and they are let loose for the day. On this day, they are permitted to pasture in any field without restraint. On returning from the river verybody takes a special bath and the younger people seek the blessings of their elders. In the evening, when the cows return home, they are tied with new ropes (Pogha) and are entertained with cakes especially prepared for them. Some people also light oil lamps and incense in the cowsheds to ward off mosquitoes and illness.

On the next day, called Manuh Bihu, special dishes made of flattened rice, curds, and jaggery and sweets are prepared and eaten. On this day 'Bihu Husori' is formally inaugurated at the Namghar (Prayer hall).

The Bihu is a community festival. People visit each other’s houses, distribute sweets to their neighbours and hold grand feasts. Rongali Bihu inspires unbounded joy and enthusiasm expressed through Bihu dances, songs and other festivities.

The third day is called Gosain Bihu and is dedicated to the worship of deities.

On all three days of the festival, troupes of musicians and dancers visit houses and perform the Bihu dance in the open. The young boys and girls wear new clothes on this day and after enjoying the special preparations of the Bihu, spend the time in egg fight ('Koni Juj'), singing songs of love and romance. Such gatherings are called "Mukoli Bihus" (Open Bihus). The songs are very popular among all sections of the people. The folk songs associated with the Bohaag Bihu are called "Bihu Geets" or Bihu songs. Young men and women perform bihu dances and sing to the accompaniment of drums and pepa, a flute made of buffalo horns. Fairs are organized at different places and the mood of festivity is present everywhere. Where available the girls decorate their long hair with Ko-pou flowers.

On the last two days of Bihu, as a mark of respect a gamocha (a traditional Assamese hand-woven cotton towel with red designs with a white background) is given to elders and their good wishes are sought in return.
Gamosa forms an integral part of the Bohaag Bihu celebrations as the male danseuse wears them on the head as well as on the body. The tradition of presenting a Gamosa is also practised while welcoming any guest during anytime of the year.Japi – a traditional Bamboo hat with colorful designs is another item which forms an integral part of the Bihu celebrations and is also symbolic of Assamese Culture. A more simple bamboo Japi is normally worn by the farmer while cultivating in the field protecting oneself from the sun and rain.

In the cities due to the pressure of of modern day life the modes of Bihu celebrations have changed to a great extent. Mukoli Bihus are not common any more. In towns and cities, there are well-organized  Clubs, Bihu committees and  Associations which organises Bihu festivals where professional and amateur groups perform.  Various contests are also organized where young men and women compete in Bihu dancing and the best dancer of each category is awarded the title. But, in most of the rural areas its originality and emotions of Bihu are very much alive.

Besides the Assamese, various tribes in the state, too, celebrate Bihu. The Bodos observe it as Baisagu, Karbis as Johang Pula, Deoris as Bahagiya Bihu, Rabhas as Nava Barasa, Tiwas as Bisu and Tai Phakes as Pasni Bihu.

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See also Magh Bihu (January) and Kati Bihu (October).