Why do we have an extra moon? Well let us explain:

The standard SOLAR calendar, introduced by Julius Caesar in 45 BC. established a 12-month structure. He named all the 12 months after himself, his nephew, important gods, festivals, and numbers in his time.
The 12 month calendar caused dates to drift into other months as centuries went by- So later a leap year, in february, was added in order to maintain the “correct dates”