Mercury has been known since ancient times. There is no specific date of discovery as it was observed by ancient civilizations.

Here's what we know:

* Ancient Sumerians: Evidence suggests they observed Mercury as early as 3000 BC.

* Ancient Babylonians: They also had detailed observations of Mercury, recording it as "Nabu" in their astronomical records.

* Ancient Greeks: They named the planet after the Roman messenger god, Mercury, due to its rapid movement across the sky.

So, while we don't have a specific date of discovery, Mercury has been recognized and observed for thousands of years.