Why did the months of the year change from 10 to 12?

The months of the year originally changed from 10 to 12 due to the Roman calendar reform. The early Roman calendar, attributed to Romulus, included only ten months, with the year beginning in March. Later, around 713 BC, King Numa Pompilius added January and February to align the calendar with the lunar year, resulting in the 12-month system we use today. This adjustment aimed to better reflect the solar year and improve agricultural planning.