I've heard there's a superstition in western culture to consider Tuesday bad luck. Why is that?

Native speaker’s answer
Rebecca
This is an older superstition that is quite uncommon these days, but it was stronger in the past and has an ancient origin. The ancient Greeks and Romans considered Tuesday as a day dominated by the god of war (Ares for the Greeks and Mars for the Romans). The medieval city of Constantinople, which was considered the greatest city in the world, was captured only twice despite being attacked many times. Both times it was captured were on a Tuesday, which strengthened the superstition that Tuesday is a day of war and bad luck. In Spanish-speaking countries, the belief is stronger because the Spanish word for Tuesday "Martes" comes from Mars, the Roman god of war.