Can a player activate a reaction card when their opponent ends their turn without doing anything?

Ruling: No, you do not get priority when your opponent ends their turn, so you cannot activate a reaction card at that time.

Nuances:

• If an effect triggers at end of turn (like Targon's Peak readying runes), opponents can only respond during the original trigger or action that created the delayed effect, not when the end-of-turn trigger itself resolves.