Panguni Uthiram is a day of importance to Tamil Hindus. This festival falls on the day the moon transits in the asterism or Nakshatram of Uttara-Phalguni or Uthiram in the twelfth month of the Tamil solar calendar that is Phanguni (March–April). It is the full moon of the month of Phanguni.
For Panguni Uthiram, this month is special because the Uthiram Nakshatram which coincides with the full moon. This full moon signifies the marriage of Parvati and Lord Shiva, Murugan and Deivanai, and Aandaal, which is also known as Kothai and Rangamannar took place.
On Panguni Uthiram, Narayana marries Komalavalli Naachiyar and gives his Kalyana Kola Seva to his Bhakthas. According to Ramayana it is on this day and star that Sita's marriage with Lord Rama was celebrated. From Brahmanda Puranam we learnt that on Panguni Uthiram every holy water joins Thumburu teertha (also spelt as Tirtha) is one of seven sacred tanks in Tirupati Tirumala.
The day is intended to underline the glory of grahasta dharma (or the married life of a householder). The Almighty manifests in the marital state as Uma Maheswara, Sita Rama, and Radha Krishna - despite his changelessness, sans childhood or youth or old age. The Lord is indeed a Nitya Kalyana Murthi.
The devotees flock in hundreds and thousands to the all Murugan temples during the Panguni Uthiram festival, which is celebrated in March every year. It is the Jayanti (Day of Incarnation) of Lord Ayyappan.
Panguni Uthiram is also an important festival of Lord Subramanya as it is on this day that Sri Deivanai married Lord Subramanya. On this day Goddess Mahalakshmi incarnated on earth from the ocean of milk (after the ocean was churned by the Gods and the demons) and hence it is celebrated as Mahalakshmi Jayanti. On this day Goddess Parvati in the form of Gowri married Lord Siva or Mahadeva in Kanchipuram and hence this day is also celebrated as the Gowri Kalyanam day.
Phalgun Poornima day is also Holi in the Braj region, which includes locations traditionally connected to the Lord Krishna. Some of the places of this region are Mathura, Vrindavan, Nandagaon, and Barsana, Kumaoni (Uttarakhand).