Can you activate a planeswalker's loyalty ability during your opponent's turn?

Loyalty abilities are a special category of activated abilities with a strict timing restriction. According to CR 606.3, a player may activate a loyalty ability of a planeswalker they control only during their own main phase, and only when the stack is empty. This means you cannot activate them on your opponent's turn under normal circumstances.

This restriction exists because loyalty abilities don't use the standard '{cost}: {effect}' timing rules for activated abilities. Instead, they have their own dedicated rule (CR 606.3) that locks them to your main phase with an empty stack — essentially the same window as when you can play a sorcery.

Additionally, CR 606.3 also states that a loyalty ability can only be activated once per turn. So even on your own turn, you can't activate multiple loyalty abilities of the same planeswalker in the same turn.

Example: Your opponent attacks with a creature during their combat phase. You control Liliana of the Veil. Even though you might want to use her '-2' ability to force a sacrifice, you cannot — it is not your main phase and the stack is not empty, so CR 606.3 prevents activation entirely.

Note: Certain cards or effects can grant the ability to activate loyalty abilities at other times (e.g., 'you may activate loyalty abilities of planeswalkers you control on any player's turn'), but absent such an effect, the answer is always no.