Mensis Ianuarius or Januarius ("January") is the first month of the ancient Roman calendar, from which the English name of the month derives. It was followed by Februarius ("February"). In the calendars of the Roman Republic, Ianuarius had 29 days. Two days were added when the calendar was reformed under Julius Caesar in 45 BC.

Mensis Ianuarius or Januarius ("January") is the first month of the ancient Roman calendar, from which the English name of the month derives. It was followed by Februarius ("February"). In the calendars of the Roman Republic, Ianuarius had 29 days. Two days were added when the calendar was reformed under Julius Caesar in 45 BC.