This is impossible to determine, since Mercury has been known and recorded since antiquity.
Mercury the planet - the earliest known recorded observations date from around the 14th century BC in Assyria. Mercury the element - Mercury was found in Egyptian tombs that date from 1500 BC. It was also known to the ancient Chinese.
pure mercury is found in volcanic rocks usually found in spain.
The discovery and date of aluminum
the ancient Chinese and the egyptians
Saturn has been visible as long as mankind can remember, so it does not have a discovery date.
1965
1808
1774
1782
1518 if you think of "discovery" as being found by Europeans.
convection currents discovery date