When must a son begin to perform amavasyai and masa pirappu tharpanam-s after ascension to Paramapadam of that person’s father? A famous upanyasakar says that for those who have only daughters as their offspring (people with no sons), rituals such as masikam and shraddha need not be performed; kindly clarify.

After one’s ascension to Paramapadam, once sapindeekaranam is done, amavasyai or masa pirappu tharpanam, which ever comes first, can be commenced. If sapindeekaranam is on the day of a tharpanam, after sapindeekaranam and sodakumbham, the tharpanam for that day must be commenced on that day itself.
If the deceased has daughter and no son, daughter’s son (dauhtira) must perform all the riutals.