The end of the year on December 31st is a result of a long historical evolution, not purely scientific fact. Ancient Romans initially began their year in March with a 10-month calendar. King Numa Pompilius added January and February around 700 BCE. Julius Caesar, with astronomical help, reformed the calendar in 46 BCE, establishing a 365-day year and setting January 1 as the start, thus making December 31 the end. This Julian Calendar was later refined by Pope Gregory XIII into the Gregorian Calendar, which the world uses today, maintaining December 31 as the year's conclusion.