There isn't a specific date for when this happened, as the naming of planets and celestial bodies was a gradual process tied to the development of astronomical observation and mythology.
It's important to remember that:
* The Romans adopted much of their mythology from the Greeks: The Roman god Mercury is equivalent to the Greek god Hermes, who was also associated with the planet.
* The planet was named for its speed: Mercury moves across the sky faster than any other planet, a trait aligning with the god Mercury's role as a messenger.
So, while we can't pinpoint an exact date, it's safe to say Mercury was named after the Roman god Mercury during the early development of Roman astronomy and mythology, well before recorded history.
