While we don't know the exact date, it was likely named during the ancient Roman period, as the planet was named after the Roman god of the sky and thunder, Jupiter.

Here's why:

* Ancient Observations: People observed the planets for thousands of years.

* Roman Influence: The Roman Empire had a significant influence on astronomy and naming conventions.

* Mythology: Jupiter's name reflects its prominence in Roman mythology.

So, while we don't have a specific year, we can say Jupiter was named in ancient times, likely during the Roman era.