Pournami means full moon day, and during that day, we can witness the full moon with full of powerful radiance, and most of the Pournamis are celebrated as festival days, since lot of festivals falls on that auspicious Pournami days. During Pournami day, we can also do auspicious acts, like opening of new business, performing Annadanam to the poor and the needy, performing marriages and visiting temples like Arunachaleswara Temple, Thiruvannamalai and SundaraMahalinga Swamy Temple, Sathuragiri which are situated in Tamil Nadu.
It is also very essential to worship Lord Chandra Bhagavan during each and every Pournami day, since he shines with full of brightness on Pournami days, and also he is the one who controls our mind. By worshipping him, our mind would not get diverted here and there, and we would get complete peace of mind, through the grace of the holy Chandra Bhagavan.
Pournami which occurs during the Tamil month, Chittirai, is known as Chitra Pournami, and that auspicious day is dedicated to Lord Chitra Gupta, who serves as the divine accountant in the Yama Loka. Worshipping Lord Chitra Gupta on Chitra Pournami by visiting Shiva and Vishnu Temples would reduce the severity of our bad karmas, and our small, small bad Karmas would be written off by the Divine Accountant, Sri Chitra Gupta.
Pournami which occurs during the Tamil month, Karthigai is considered as very auspicious and it is dedicated to Lord Muruga and Shiva, and worshipping them on that auspicious day, would fetch good results in our lives.
Guru Poornima would occur on the Tamil month of Aadi, and this festival is dedicated to the holy Sage, Sri Vyasa Bhagavan. Worshipping the Holy Gurus like Dakshinamoorthy, Guru Raghavendra and Shirdi Saibaba on that important day is also considered as a good act.
Vat Purnima also known as Ma Savitri Purnima is celebrated on the full moon day in the month of Jyeshta. During that day, Hindu Women used to pray for the well-being of their husbands and they would also perform puja to the deities. This Purnima Festival is celebrated in order to honour the chaste and the noble woman Ma Savitri, the lovely wife of Satyavan, who prayed to Lord Yama, and restored the life of her husband Satyavan.
Buddha Purnima,occurs on the Tamil month of Vaikashi. Worshipping Lord Buddha and reading his teachings on that auspicious day would enlighten us.
Holi the festival of colours, would occur on the Tamil month of Panguni, and it would be mostly celebrated by the North Indians, by throwing colours powders on one another, in order to express their happiness on that auspicious festival day. Once the great Bhakta Prahalada had killed his wicked aunt Holika, and the day which he killed her is celebrated as the Holi festival day.
Likewise there are lots and lots of festivals celebrated on Pournami festival days.
Worshipping the Navagraha deities during Pournami days, would fetch their kind blessings, and all of our Navagraha doshams would be reduced at least to some extent.