Panguni refers to last month in the Tamil calendar and Uthiram refers to Nakshatra or star that is ruling. It falls on the day the moon transits in the asterism or Nakshatra of Uttara-Phalguni or Uthiram in the twelfth month of the Tamil calendar i.e. Panguni.
How the date is decided annually
It is the Purnima or full moon of the month of Panguni.
Significance of the festival to Hinduism
On Panguni Uthiram, in all places where Lord Subramaniam has a temple, his devotees carry in a Kavady the requisites for prayer for him, in fulfilment of vows. Such vow fulfilment by devotees carrying Kavady is a special feature of Subramaniam temples wherever they happen to be. Devotees also celebrate the marriage of Lord Shiva and Mother Parvati on this day. On this very day, the marriage between Devaseenaa and Lord Muruga is also commemorated.
How to observe/practice
Devotees conduct with full enthusiasm the rituals of the Divine marriage of the Gods and Goddesses. Devotees must early that day and meditate about the amiable Lord Shiva in the wedding posture with Goddess Parvati. The devotees must then take a bath and wear clean clothes, before worshipping their deity. After observing a strict fast all day, the devotees can eat Prasad in the form of sweet Payasam (a dish prepared from milk) and fruits together with other devotees of Lord Shiva. Only one meal can be eaten by the devotees on that day. Devotees also carry Kavady on this day.