Forebruary, an endless wall calendar
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.