You don't need to reflect on the complexity of the vertebrate eye to invoke the existence of the deity, but there are some pretty strange 'coincidences' out in the natural world. How is it that the perceived diameter of the sun and of the moon from the surface of our blue planet are almost exactly the same? Did an early eclipse trigger the spark than became life on the planet? Has God arranged it so that his scholarly priests can cow their rebellious flock by predicting that the sun will go out . . . now? Answers on a postcard (or a comment below) please.
The sun and the moon and the rotating earth are in all other respects annoyingly out of kilter. Wouldn't you think that the day (one turn of the earth) would be exactly 1/30th of the the month (one turn of the moon) and that would be exactly 1/12th of the year (one turn round the sun)? Well it's not and it jiggers us all up in ways great and small. But then again, maybe the niggling irregularity of the tides being scarcely predictable (by the per gene product for example) may have pushed some poor fish out of water to become the first effective tetrapod. One could, and The Blob will, go on and on about this but we'll pause today and reflect on the discrepancy between the lengths of the day and the year.
The Siderial Year is the time taken for the earth to make one circuit of the sun with respect to an arbitrary fixed point (pick any star: Sirius is particularly bright). This is just over 365 and a quarter days: 365d 6h 9m 10s. This is different from the Mean Tropical Year (365d 5h 49m 16s), which is when the Sun returns to shine down the orifice at Newgrange, by about 20 minutes. Depending on how precise you need/desire to be, you must adjust the number of days in the year or suffer having key dates (like Easter, the foundation of The City or the Battle of Clontarf) slip through the seasons. In 708 AUC Julius Caesar rejigged the Roman Calendar to alternating months of 30 and 31 days and a short late winter month Februarius of 28 days (most years) or 29 days (every fourth year). This created a year that was 365.25 days, was easy to calculate and was for most practical purposes a pretty good year.
370 years later, the 1st Council of Nicaea was convened, by the first christian Emperor Constantine I, to decide when Easter was to be celebrated. Tradition clearly had the crucifixion happening at the Jewish Passover but the Council agreed to a) have all Christians celebrate the feast on the same day and b) no longer rely on Jewish authority for determining the date. Those precepts were not enough to unite the whole church because the RC and the Orthodox churches still have slightly different methods of calculating Easter - this year my Greek room mate was celebrating Easter 5 weeks after the rest of us but next year both churches will pick the same date (20th April, as you ask).
1500 years later, the Council of Trent was convened (in Trento, in the Italian Alps not Nottingham!) to wrench Easter back to the time when the Holy Fathers at Nicaea had said it should be. It had drifted because the Julian Calendar's pretty good adjustment had been accumulating a drift of 10 minutes a year for 1.5 millennia, so was then about 10 days off. It was clearly important because the Council of Trent was actually Councils: the senior clerics, their scribes and mitre-carriers, met for 25 sessions over a period of 18 years and three papacies. The final decrees were signed by more than 200 prelates. That's about 8 person years for filibuster and preening and a couple of days of actually getting stuff done. Although I agree that there was more on the Agenda than the calendar.
There were two issues: a) get the calendar back on track b) make sure the drift wouldn't happen again. Both problems was solved by a doctor from Calabria called Luigi Lilio. He proposed reducing the number of leap years from 25 per century, Julian style, to 97 per 400 years: century years 1700, 1800, 1900 would not have a leap year but millennial years 1600, 2000 would. He also suggested that the first problem be solved incrementally by omitting the next 10 leap days. Christopher Clavius (born Christoph Clau or Christoph Schlüssel) took up cudgels for Lilio after the latter's death writing his arguments into an 800 pager. I don't know how these guys would have coped on Twitter. Clavius argued for a one-fell-swoop adjustment of the Calendar and the next-Pope-but-one Gregory XIII (born Ugo Boncompagni) agreed.
Accordingly in 1582 4th October was followed immediately by 15th October and Cardinal Alessandro Farnese (b 5th October 1520) didn't have a birthday that year - he must have been a bit hacked off.