Calendar Days

Here's a question that I'm having some difficulty pinning down:

Why do some months have different number of days than other months?

So far I've tracked it back to following the lunar cycle of 29.5 days. The Greek Attic Calendar roughly followed this with alternating months of 29 and 30 days. I'm sort of stumped on the exact history of how it went from that to the current 28 days in February. Still looking.