[halt i am reptar]

I think originally, the months were lunar. Up until about 1,800 years or so ago, the calendar year started in March, which makes the months SEPTember, OCTober, NOVEMber, and DECEMber sensically named. I forget what July and August were originally, but either way those months were renamed for Julius first, then Augustus later. To properly honor the leaders, whoever made the change added one extra day to each of those months. Thus, the end of the year, February, came 2 days earlier. This still does not explain other months with 31 days in them, but offers some sort of insight.

Of course, this is just an explanation, not a justification