Poornimaa is the last day of the moth on Hindu Lunar calendar; or the last day of Shukla Paksh. Thus there are 12 Poornimaa every year. Although all Poornimaa are auspicious, but some are more auspicious than others. Many people keep Poornimaa Vrat on every Poornimaa. On that day they do Satyanaaraayan Kathaa also. The following four Poornimaa are supposed to be very auspicious as four Amaavasyaa.

Vaishaakh Poornimaa - Buddha Poornimaa / Ashwatth Poornimaa
This Poornimaa is very holy specially for Bauddha people, because Buddha was born on this day. This is called Ashwatth Poornimaa also. This day Peepal tree is worshipped.

Aashaadh Poornimaa - Guru Poornimaa
On Aashaadh Poornimaa. As its name suggests, it is the day to worship Guru. Worship Ved Vyaaas Jee, Dattaatreya Jee, and Brihaspati Jee. Start Chaaturmaasya Vrat also this day.

Aashwin Poornimaa - Sharad Poornimaa Vrat
Aashwin Poornimaa or Sharad Poornimaa is the most important Poornimaa among all Poornimaa for Hindu. It is believed that this night the Moon is very clear and its rays are full of Amrit. This day people keep fast, do Darshan of Moon and break fast. Chivadaa (or Chaulaa, or from which people make Pohaa, or beaten rice) is soaked in milk and yogurt separately and made rice Kheer (rice pudding) also. They are kept under the moonlight for the whole night so that they can soak the Amrit coming from Moon rays. Next day morning they are eaten.

Kaarttik Poornimaa - Vrishotsarg Vrat
(1) This day ends the Kaarttik Snaan. Who do Snaan the whole month of Kaarttik end their Snaan this day. People go for Gangaa Snaan also. After this Snaan they go to Snaan in Maagh month. The Snaan again starts from Maaagh month.
(2) This day Bheeshm Panchak Vrat also end.
(3) On Kaartik Poornimaa, do Vrishotsarg and do night Vrat. This gives Vratee Shiv Lok.  ( Agni Puraan, 21 )
(4) This day is celebrated as Guru Naanak's birthday also by Sikh people.

Phaalgun Poornimaa - Ashok Poornimaa Vrat - Holee
On Phaalgun Poornimaa - Holee. Worship Bhagavaan Varaah and Bhoo Devee. By doing this Vrat Vratee attains pleasures and Moksh.
Agni Puraan, 21

[I have done a special Poornimaa Vrat - "Paanch Naaj Pachchees Poono" - means five types of grains are eaten on five Poornimaa each, thus completing 25 Poornimaa Vrat. These grains were - wheat, rice, split yellow Moong Daal, barley and Kootoo (Phaaphad flour, or buckwheat flour). On 26th Poornimaa its Udyaapan is done. A Braahman is fed and some clothes and Dakshinaa are offered to him. This Vrat is normally started from Sharad Poornimaa and there is no restriction of salt in this Vrat, except some non-fasting foods, such as Moolee, carrots, ginger etc. Mostly potato, Arabee (yam), pumpkin (Kaddoo) are eaten in vegetables and any Daal (Pulse) may be eaten with rice except red Masoor (red lentils)]