The calendar you use today is a Roman calendar. The twelve months, the seven-day week borrowed from Near Eastern sources and transmitted through Rome, the numbering of the years from a fixed point that eventually became the Christian era — all of these are features of the system that Julius Caesar reformed in 46 BC and that the Catholic Church adjusted in 1582 with modifications so minor that most countries now use what is, in its essentials, the calendar Caesar commissioned. You wake up on a Tuesday in October because a Roman dictator in the first century BC decided to align the civil year with the solar year, and his solution was good enough to last two thousand years.