Why is Mars called "Mars"?
"Mars" is what Romans used to call the corresponding planet (now known to be the fourth planet from the Sun) in Latin. For the Romans, Mars was initially a god of fertility, agriculture, and vegetation, but later became the god of war. The Romans considered Mars favorable to their wars, and consequently held this god in high esteem unlike the Greeks who thought of the same god (called Ares by them as the son of Zeus and Hera) as bloodthirsty. Interestingly, the word Mars could be from the Etruscan (an ancient civilization - in what is modern day Italy - that greatly influenced the Roman civilization) word "Maris". Maris signified the same god for Etruscans who used a language distinct from the Indo-European group of languages, and was called "Marmar" in Old Italic languages.
Interestingly, "Tuesday" is also linked to Mars, though it was initially associated with the Teutonic god Zio (Old High Germanic). This led to the Modern English "Tuesday" literally meaning the "day of Zio". However, having identified Zio as Mars, Romans called the day "dies Martis" (day of Mars) in Late Latin, which then led to the French "Mardi" as presently used. That association exists throughout modern Indo-European cultures. Similarly, the Romans dedicated the third month to Mars and called it "mensis Martius" in Latin meaning "the month of Mars," which came to be known as March in modern English. This association of the third month with the god entered the (modern) Gregorian calendar via the Julian calendar.