Our calendar was developed in ancient Rome, and originally it had a ten month year. From March to December, the calendar resembled ours, but January and February were missing. The Romans left 61 days in an unnamed period between December and March. Legends claim that King Numa Pompilius added January and February  to the calendar around 700 BC.
The missing months are a mystery. Ancient people liked to begin their months on the day of a new moon, but the moon’s cycles did not correspond to the change in seasons. No one could reconcile seasons with lunar months, and perhaps the Romans decided not to try. Better to take a mid-winter break instead. Then, as spring approached, they could watch for a new moon and proclaim it to be March 1.