Ianuarius

thumb|upright=1.5|Drawing of the fragmentary Fasti Antiates, a pre-Julian calendar showing Ianuarius (abbreviated IAN) at the top of the first column

', ("January"), or in full , abbreviated ', was the first month of the ancient Roman calendar, from which the Julian and Gregorian month of January derived. 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 BCE.