For those of you paying attention, Easter came a tad late in the
season this year. But what if it got so bad it was celebrated in May?
That was the problem faced by leaders of the Catholic Church in the late 1500's. The calendar they were using, initially proposed as a reform by Julius Caesar a millennium and a half before that, didn't exactly line up with the actual length of the year. Sure, it was good enough for a couple hundred years, but over time errors crept in, and soon enough it was shorts-and-sandals weather during the big Spring party.
Pope Gregory XIII was sick of it, and so - another reform. Every 4 years would be a leap year (a feature of the older Julian calendar), unless the year was divisible by 100 (then no leap year) unless it was divisible by 400 (then leap year anyway). These changes to the leap schedule shortened the average length of the year by almost 11 minutes, meaning only minor corrections need to be made every once in awhile to keep everything synched up.
The Gregorian calendar is a little weird, but it does the job.