Askipedia.com - Why are there 12 months in a year?
In order to keep the seasons from shifting dramatically and confusing the animals there were occasionally leap years, just like ours (sort of). Instead of adding one day in February, an entire month as added. This leap month was known as Mercedonius. For some strange reason during a leap year February (known then as Februarius) was shortened to 23 or 24 days, the leap month began at the end of Februarius.
