Does a landfall trigger fire if you play a land during an opponent's turn using a flash effect?

Landfall is a triggered ability that watches for a specific event: a land entering the battlefield under your control. The Comprehensive Rules define triggered abilities as those that begin with 'when,' 'whenever,' or 'at' and trigger automatically when their condition is met (CR 603.1). Landfall uses 'whenever a land enters the battlefield under your control,' with no restriction on which player's turn it occurs.

Playing a land normally is restricted to your own turn and your one land-per-turn limit (CR 305.2), but certain effects grant the ability to play lands at other times — for example, effects with flash-like wording such as 'you may play an additional land this turn' combined with an instant-speed opportunity, or cards like Exploration with a flash enabler. If such an effect allows you to legally put a land onto the battlefield under your control during an opponent's turn, the landfall trigger fires just as it would on your own turn.

The game doesn't care about whose turn it is when evaluating whether a triggered ability's condition was met. CR 603.2 states that triggered abilities trigger whenever the event described occurs, and CR 603.3 says the triggered ability is put on the stack the next time a player would receive priority after the triggering event. Turn ownership is simply irrelevant to the trigger condition.

Concrete example: Your opponent attacks. You control a Ruin Crab and have an effect that lets you play a land at instant speed (e.g., your opponent controls Teferi, Time Raveler's static ability is absent and you have a 'you may play lands as though they had flash' effect). You play a basic Island during your opponent's combat step. Ruin Crab's landfall ability triggers immediately, and each opponent mills three cards — even though it's not your turn.

In summary, landfall triggers are entirely event-based, not turn-based. Any legal method of getting a land onto the battlefield under your control — on any player's turn — will fire the trigger (CR 603.1, 603.2).