But, as we now know, the #JulianCalendar's mean year is about 11 minutes 15 seconds longer than the #TropicalYear, and by the 4th century AD, the dates of the #solstices and #equinoxes had drifted from their official dates by several days.

In the 16th century, #Pope Gregory XIII used his authority as #PontifexMaximus to reform the calendar again, because that is something #popes can do.

See, declaring a #LeapYear was actually the decision of the #PontifexMaximus, which was an elected political position. If Pontifices Maximi would routinely declare a leap year if they liked the #consuls, and refuse to declare one if they disapproved, in order to prolong or contract the administrations of various consuls according to their own preferences and ambitions.

Plus, since a #LeapYear wasn't declared until late, outlying parts of the empire wouldn't know until months later.