Amavasya is new moon day in Hindu calendar. It is significant day as many rituals are performed only on Amavasya Tithi. Amavasya falling on weekday Monday is known as Somvati Amavasya and Amavasya falling on weekday Saturday is known as Shani Amavasya.
All Amavasya days are appropriate to perform Shraddha rituals to appease ancestors. Amavasya day is also appropriate day to perform Kalasarpa Dosha puja.