If we had 13 months, all of them would have exactly 28 days. We would need and extra day in order to complete 365 days, but we could make it a special case (the first day of the year, for instance) not belonging to any month or week. Then all months would be alike, every 1st, 8th, 15th, etc, would be a Sunday, and so on. We would never need a calendar again.
Stupid stuff like February having only 29 days is pretty funny and those 7 days of December 25th to January 1st is perfect because its an exact week.
It would be too boring if every month was the same.
Could we have fun stuff like `April Showers Bring may Flowers' if every month was just the same 28 days? I'm sure we could, but everything would be the same and that's boring.