Sikhs celebrate Kartik Purnima as Guru Parb and Nanak's birthday, but the Hindus celebrate it as a day when God incarnated himself as 'Matsya' or Fish. Vishnu took the Fish Incarnation to save Vaivasvata, the seventh Manu and the progenitor of the human race from the Deluge.

Charities done and piety observed on this day are believed to earn high religious merit. Bathing in the Ganges, or in other holy water is considered to be of special religious significance. People keep fast, practice charities and meditate on God.

It is also believed that Shankar killed demon Tripurasura on this day and so he is also called Tripurari. Shiva is worshipped on this occasion and giving away of a bull (bull is mount of Shiva), as a gift to a Brahmin, is thought to be of great religious significance. Big cattle fairs are also held on this day at various places. For example, the cattle fair held at the sacred Pushkar Lake, near Ajmer, is a great draw, which transforms the scene into a seething sea of colour and gaiety, tempered by the presence of a huge number of devotees. Thousands of camels, cows, bulls, buffaloes, sheep, goats etc., are brought there for sale. Camel race is held and gaiety fun and merry making marks the occasion. Over a hundred thousand pilgrims take bath here on this day in the sacred lake.

Kartik Poornima is also referred to as Tripuri Poornima. A bath in river Ganga and donation of a lamp hold special significance on this day. If on this day the Nakshatra is Bharani, Kruthika or Rohini then one can expect better fruits. It is said that, on this day that Lord Vishnu incarnated himself as a fish (Matsyavtaar). One who donates on this auspicious day is blessed with the fruits of 10 religious sacrifices (Yagya). One this day the observer should worship the 6 Kruthikas, Shiva, Sambhuti, Santati etc at time of moonrise. One who ceremoniously donates a buffalo at night, is blessed and achieves highest state of celibacy (Shivapad). One should observe a fast on this day and should contemplate upon God. On doing so one is blessed and he/she also attains the Suryalok. If one donate a ram made of gold on this day one is freed from problems of Grahadosh (problems and curses of planets). There is a custom even of donating of Ksheer Shayya (an ocean). One should fill the dish with milk and should disburse a fish made of gold or silver in the dish an donate it.