This article generally discusses the early Roman or 'pre-Julian' calendars.

Nevertheless, it is known that the pre-Julian calendar could be substantially out of alignment with the Julian calendar.

The year 146 BCE was a year of the pre-Julian Roman calendar.

Februarius was the only month in the pre-Julian calendar to have an even number of days, numbering 28.

Year 46 BC was the last year of the pre-Julian Roman calendar.

Year 235 BC was a year of the pre-Julian Roman calendar.

For these reasons, the last years of the pre-Julian calendar were later known as "years of confusion".

Although the new calendar was much simpler than the pre-Julian calendar, the pontifices initially added a leap day every three years, instead of every four.

Introduced by the consul M. Acilius, this law probably authorized the pontifices to decree an intercalary period in order to prevent seasonal drift in the pre-Julian lunar calendar.

The Mensis Intercalaris began on this day or the following day in intercalary years in the pre-Julian calendar.