In the song, it's ten lords a-leaping. Outside that tune -- where people live remarkably full lives without hens, swans, geese, turtle doves, and partridges underfoot -- it's seven billion or so left a-leaping today, almost the whole planet now doing it, and doing it by decree, called a bull, by a pope, long ago. So, Happy Papal Bull Day!
Pope Gregory XIII gave his name and breath of life to the Gregorian calendar we now automatically use, back in February of 1582. It built its fanbase from there, eventually becoming the accepted secular calendar for the western world -- slow adoption and Protestant Reformations not withstanding, in which papal edicts were given cold-shouldered snubs.
The change, way back when, 430 years ago, was trying to right a wrong in the old way of measuring the progression of days, using the Julian calendar, named after Julius Caesar who gave it a debut in 45 BCE. As you can see, we are in the Way-Way Back Time Machine, now.
One key problem was that time was wonky and way off, off by 11 minutes or so. This may seem like a trick problem with apples and ocelots, or the average number of feet in a liter, asking about minutes on a calendar, but here goes:
Advocates of Julian calendars thought the time between vernal equinoxes was 365.25 days, right on the button. Oops: It's actually shorter than that, by 11 minutes or so. No big deal, but if you are the Roman Church, likely to take a verrrry long view of things, you'd notice spare days starting to pile up. Every four centuries you've got three extra days on your hands, seemingly popped out of the void, no idea where they came from.
One supposes they could have left it alone at that point, periodically declared a special Festival of Bonus Days every 400 years, but you know how that would work out: You give one generation a three-day run of bonus partying days, sunup to sunset, next thing you know, they've all got to have one. Good luck to saying, at that point, in parental voice, "Do as I say, not as I do," when it comes to three days of bonus partying held in the balance and sway.