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.