Mars was the Roman god of war and fighting, the son of Juno (all by herself, with no dad). People in the Roman Empire thought of Mars as being similar to the Greek god of war, Ares, but Mars was a popular god, who fought to bring peace and safety. Greek people felt Ares was more about destruction and chaos. In the early Republic maybe the Romans felt this way too, because early temples to Mars were outside the city limits of Rome, out in the fields. But the emperor Augustus brought Mars into the very center of Rome, building Mars a temple in his forum downtown.
Our word for the month of March comes from Mars, because during the Roman Republic March was the month when the Roman army assembled on the Campus Martius – the field of Mars – outside of the city of Rome, to get ready for the summer’s wars against their neighbors.
One of the days of the week is also named after Mars – Tuesday. The Romans called Tuesday dies Martialis – the day of Mars – and in Italian Tuesday is still Martedi. In French it is shortened to Mardi, which is where our “Mardi Gras” comes from. But in English, people translated it using the name of the German god Tyr, who was also a god of war, so we say “Tuesday”.