Question

The birds and frogs rejoice and sing, "Ding-dong-tock! Nag is dead—dong! Nagaina is dead! Ding-dong-tock!" In what part of the plot does this event take place?

Answer 1

This event takes place after Rikki-tikki killed Nagaina. The coppersmith bird, that makes a "noise exactly like  the beating of a little hammer on a copper pot", is telling the whole garden that Nagaina is dead.