Rules for Sraddha - Verse 33-34

After procuring the required money for the Sraddha ceremonies and choosing a Brahmin of particular qualities for the purpose, the rites are to be done on the occasions of Vyatlpata (the day of full-moon when it falls on a Monday), Ayana (the solstices) Visuva (the equinoxes)eclipses of the sun and the moon, and of all signs of zodiac (Rasi) at the time each is entered into by the sun.