Can you use reaction/action cards on your opponent's turn whenever you want (like at end of turn), or only during specific windows like showdowns or in response to spells/effects?

Ruling: You cannot use cards whenever you want on your opponent's turn. You only get priority when your opponent does something that gives you priority, such as during a showdown or when they play a spell.

Nuances:

• You do not get priority at the end of your opponent's turn
• Triggering an end phase ability (like Dazzling Aurora) creates a chain that opens a window where other players can respond when they get priority
• In 2v2 matches, players may play spells during their teammate's turn