The 15th day of the dark fortnight Lunar Calendar. The New Moon day. There are three divisions of Amaavasyaa - Sineevaalee, Darsh, Kuhoo. The last Prahar of 14th day and 8 Prahar of Amaavasyaa - these 9 Prahar time period is considered the decreasing period of Chandramaa (Moon). [8 Prahar = 1 day and night]. Among these 9 Prahar, in the first 2 Prahar there exists some Kalaa of Chandramaa, thus it is called Sineevaalee; and in the last 2 Prahar no Kalaa of Chandramaa exists, that is why it is called Kuhoo. The middle period of 5 Prahar is called Darsh. See also Kalaa. 
(Naarad Puraan, p 109)

There are five kinds of Amaavasyaa -
(1) Ordinary Amaavasyaa - Which comes every month;

(2) Somvatee Amaavasyaa - Whichever Amaavasyaa falls on Monday is called Somvatee Amaavasyaa. People take dip in Gangaa River this day.

(3) Shani Amaavasyaa - Whichever Amaavasyaa fall on Saturday, is called Shani Amaavasyaa. On this day people worship Shani Dev, Peepal tree etc.

(4) Maunee Amaavsyaa - This Amaavasyaa falls in the month of Maagh. People try to keep Maun (silence) on this day for as long as they can.

(5) Pitar Amaavasyaa - Or Pitar Samokinee Amaavasyaa. This day comes every year in Aashwin month at the end of the Pitra Paksha. This day is good for Pitar Shaanti; for reducing and removing the effects of Tantra and Mantra; worship Shani (Saturn), Raahu and Ketu as they are malefic planets; to get relief from long-term illness or depression. This day people do Tarpan for all their PitarFor Pitar Shaanti do Pitar Tarpan, or read Geetaa's 7th chapter, or worship Peepal tree. Worship should be done by offering sweets and Deepak. (see also   Shraaddh)