Originally Posted by American Maid
My guess would be that it is after the "huge hit"--the attack of the black ship, the scene with Algrim, possible death of Frigga, etc. Maybe Odin is weakened to the point that he feels compelled to name Thor king immediately.
It's stunning to think he would abdicate, though it addresses the practical issue of keeping him free to fight with the Avengers.
Why would it be surprising for Odin to abdicate? He did just that in the first movie; it's just that the Frost Giant raid interrupted Thor's coronation. The only thing that held Thor back was him needing to grow up/learn some humility, which he did in the last two movies.