The Gregorian Calendar
I honestly feel like our current calendar system is a massive missed opportunity to simplify things. 12 months a year, but 4-5 weeks in a month, and anywhere between 28-31 days in a month.
But if we had 13 months in a year, we could have exactly 4 weeks in a month. Like literally every Friday would always have the same 4 dates. Yes there would be one extra day we would need to throw somewhere, but we could just have that be like a global holiday that falls between 2 weeks.
The Gregorian Calendar
The 29th day of the 13th month. It falls outside the normal week, so it would be like Saturday the 28th, followed by a day of celebration to bring in the new year with street festivals and the like, and then Sunday the 1st. How awesome would that be?