When my father passed away, I started doing amavasai tarpanam after one year, after abdheekam. My mother passed away recently, but vadhyar has asked us to include her in the amavasya tarpanam right away. Not quite sure, why it is so. For our mother, we are only doing oona masikams, 6th month and abhdeekam next year, due to constraints at home. The monthly masikams will be done cumulatively at the 6th and 12th month functions.