Bodhayana Amavasya is the Amavasya which is observed a day prior to actual Amavasya tithi. In 2013, Bodhayana Amavasya dates are – September 4th (Wednesday), November 2nd and December 31st.
Bodhayana Amavasya of September 4, 2013 – is Bhadrapada Amavasya in North Indian Hindi calendars and Shravan Amavasya in Gujarati, Telugu, Kannada and Marathi calendars.
Bodhayana Amavasya of November 2, 2013 – is Kartik Amavasya in North Indian Hindi calendars and Ashwin Amavasya in Gujarati, Telugu, Kannada and Marathi calendars.
If on Chathurdasi day, Amavasya Thithi starts before 12 hours (30 Nazhigai) from Sun rise, then those who follow Bodhayana Sutra do Tharpana on that day. They also perform Tharpana on Chathurdasi day when, the Pratipada thithi next to Amavasya day ends within 12 hours after sun rise.
It is to be remembered that this particular Amavasya day is directly connected to the artificial Amavasya story related to Mahabharata. Pandavas along with Lord Krishna performed Tharpanam before the beginning of the war in order to make the Kauravas started the said war in an inauspicious day and time.