Define "civilization" and define "contact". If there's no one definition for either (which there won't be), then there's no singular answer to your question.

An example question which would work better: "when was the first time two rulers who ruled over a population of 1 million or more met each other personally?"

Something like that is specific enough to be answerable, assuming the answer hasn't been lost to history.

But if, as I suspect, you're looking for a good comparison to first contact in Sid Meier's Civilization games, you won't find that here.