Opportune means arriving at exactly the right moment. The phone call that comes just as you needed an excuse to leave.
It describes timing and nothing else, which is what separates it from lucky or convenient. An opportune moment is the right one to act in; an opportune arrival lands when it helps most. You will meet the negative just as often, and inopportune is the word for a knock at the door at the worst possible time. Do not confuse it with opportunistic, which is about taking advantage of a chance and carries a hint of self-interest that opportune does not.
Proto-Indo-European
*per- (fare)
Latin
opportunus
Old French
opportun
English, today
opportune
From Old French opportun, from Latin opportunus.
Etymology from Wiktionary, CC BY-SA