The Sun's name, or rather the English word "Sun", has been around for a very long time, predating recorded history. It comes from the Proto-Germanic word "sunnō", which itself is derived from a Proto-Indo-European root meaning "to shine".

So, while we can't pinpoint an exact date, it's safe to say that the Sun has been called "Sun" for thousands of years.

Important to note: While the English name "Sun" is ancient, other cultures have their own names for the star. For example, the Greeks called it "Helios", and the Romans called it "Sol".