I am constantly amazed by the complexity and beauty of the calendar. We humans are mostly stuck moving through time linearly, but we've given that line significant attention and meaning. Seasons and holidays measure our path, break up our monotony, and teach us our culture all at once.
April Fool’s Day, like practically everything else in Western Civilization, has Catholic roots. As part of the Council of Trent in 1563, it was decided that a new calendar was needed in order to properly calculate the vernal equinox. Because Easter occurs on the Sunday following the first full moon after the vernal equinox, this calculation is important in determining the highest holy day, as well as Lent and Pentecost.
In 1582, Pope Gregory XIII instituted the Gregorian calendar. It was a remarkable scientific feat. Calendars are based on solar and lunar observations, but our sun and moon don’t quite line up. The interval between successive vernal equinoxes (365.2424 days) is approximately 11 minutes less than 365.25 days. The time between each full moon is around 29 1/2 days, so 12 lunar months add up to only about 354 days. The Julian calendar organized time on a 365 day cycle, with leap years to account for the quarter day accrued every year. But over time, it still fell 10 days behind. The Gregorian calendar solved the problem by jumping ahead those 10 days, instituting leap years, and excluding leap years on century markers unless divisible by 400 (so 2000 had a leap day, 1900 did not). Like the Julian calendar, the New Year began on January 1.
Unfortunately, the Gregorian calendar went into effect post-schism and post-reformation. To this day, the Eastern Church still uses Julian dating to determine holy days. Catholic countries adopted the new calendar rather quickly, but Protestant countries wouldn’t, suspicious of a Catholic plot and following dirty, papist traditions.
England (and its colonies) was one of the last countries to make the switch. While there was the anti-Catholic sentiment, most the concern was down to taxes, annuities, and other financial matters in a year that would skip 11 days ahead (for it had fallen another full day behind between 1582 and 1750). It was only when it got too complicated doing business with foreign nations, including Ireland and Scotland, that England finally switched. However, the Calendar Act of 1750 specifically made no mention of the Gregorian calendar, but just happened to adjust the English calendar and its Easter calculation to line up with what everyone else was using.