Apparently this goes back to the ancient Romans...yet another "tradition" from a dead ancient culture we need to get rid of.
I also like this one
https://en.wikipedia.org/wiki/World_Calendar
The World Calendar is a 12-month, perennial calendar with equal quarters.[1]
Each quarter begins on a Sunday and ends on a Saturday. The quarters are equal: each has exactly 91 days, 13 weeks, or 3 months. The three months in each quarter have 31, 30, and 30 days respectively. Each quarter begins with the 31-day months of January, April, July, or October.
The World Calendar also has the following two additional days to maintain the same new year days as the Gregorian calendar.
Worldsday
The last day of the year following Saturday 30 December. This additional day is dated "W" and named Worldsday, a year-end world holiday. It is followed by Sunday, 1 January in the new year.
Leapyear Day
This day is similarly added at the end of the second quarter in leap years. It is also dated "W" and named Leapyear Day. It is followed by Sunday, 1 July within the same year.
The World Calendar treats Worldsday and Leapyear Day as a 24-hour waiting period before resuming the calendar again. These off-calendar days, also known as "intercalary days", are not assigned weekday designations. They are intended to be treated as holidays.