January and February were both created in the eighth century B.C.,
shortly after the time of Rome's founding. They were added to a calendar
that had been divided into ten month-like periods whose lengths varied
from 20 to 35 or more days. Those lengths are believed to have been
intended to reflect growth stages of crops and cattle. The winter season
was not included.
When introduced, Januarius (January) was given 29 days and put at the
beginning of the calendar year. Februarius (February) was given 23 days
and put at the end. Then, for an undetermined period shortly after
Rome's founding, months were said to have begun when a new moon was
first sighted. At some later time, month lengths were separated from
lunations and again became fixed. At that time, February's original
length was extended by five days to give it a total of 28. If you would
like to read about other nations that also added five days to their
calendars close to the same time see my 8th to 4th Century B.C. Calendar
February became the year's second month in 452 B.C., when the Decemvirs
moved it from it's original position as the last month.
According to Ovid's Fasti, February had 29 days during common years in
the original Julian Calendar.