Must the masa pirappu tharpanam be performed before the month is born? Or must it be done only after the birth of the month?

There are specific determinations for each masa pirappu tharpanam and all are based on the same calculations. If it is Shadasheeti, tharpanam is done after the birth of the month; even in this calculation, there are various factors involved. We shall look into this in detail at a later date.