Some abilities trigger "whenever you cast a historic spell." Such an ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

An ability that triggers "whenever you cast a historic spell" doesn't trigger if a historic card is put onto the battlefield without being cast.

A historic spell is a spell that is legendary, is an artifact, or is a Saga.

Lands are never cast, so abilities that trigger "whenever you cast a historic spell" won't trigger if you play a legendary land. They also won't trigger if a card on the battlefield transforms into a legendary permanent.