Pournami is the full moon day of the lunar calendar. The full moon days (Pournima) are days of great joy and always days of festive gathering.  “Pancha Parva Utsavams” or the group of five festive celebrations in Shiva temples are observed every month on five days namely Amavasya (New Moon), Pournami (Full Moon), Ravi Sankramana (Start of a Solar Month), Krishna Paksha Ashtami (8^th day of dark fortnight of the lunar month) and Krishna Paksha Chaturdashi (14^th day of dark fortnight of the lunar month). 

Each Pournami is dedicated to the worship of Lord Uma-Maheswara. The special Uma-Maheswara vrata is observed on Kartika Pournami. 

The name of each solar month is based on the nakshatra (star) that coincides with the full moon occurring that month. Each Pournami is celebrated - Chitra Pournami, Vaisaka, Ani Purnami (Karaikkal mango festival), Adi guru purnima, Avani Gayatri Japa/Rakshabandan, Purattasi Niraipani, Aippasi Annabhishekam, Arunachala Thirukarthikai, Markazhi Ardra Darshanam, Thaipusam, Masi Magham, Panguni Uttiram. On the full moon of each month, a special abhisheka is performed to Lord Shiva. The following lists the special material used for abhisheka based on the solar month - 

Generally, in any Shiva temple, the Brahmotsavam culminates on a Pournami. 

At the Tiruvannamalai, the festival of Karthika Deepam coincides with the full moon in the month of Karthigai. Here performing the circumambulation (girivalam) on the full moon of every month is considered auspicious. Following this custom, on Pournami, even at Shiva temples at the foot of other hills, devotees perform this kind of girivalam with Lord Annamalai in their hearts.

See Also:  
1. Uma Maheshvara Vratam

2. Tiruvannamalai Arunachaleshwarar Temple