Dating and Calendars
Confusion about the dates is caused by the fact that the Pilgrims used the Julian calendar, while we use the Gregorian calendar. The principal, but not the only, difference between the two is when they have leap days. Technically, the two calendars do “agree” for 1 March AD 200 through 28 February AD 300, but those dates really do not occur in Mayflower history.
The Julian calendar has one leap day every four years. Thus, the average length of a Julian year is 365¼ days. The difference with the actual length of a tropical year is small and not immediately noticeable, but it does add up over the centuries, making the Julian calendar slowly drift relative to the seasons. The Gregorian calendar fixes the drift issue, through a more complex leap day rule, to better match the actual length of a year. The leap year rule is this: years that are multiples of four are leap years, except that years that are multiples of 100 are not, and years that are multiples of 400 are leap years. The Julian calendar is named after Julius Caesar, who introduced it as a reform of the Roman calendar. The Gregorian calendar was instituted by the papal bull Inter gravissimas (24 February 1582) of Pope Gregory XIII, after whom the calendar is named, because he noticed Easter shifting relative to the seasons. At that time, the accumulated error of the Julian calendar was ten leap days. To correct for this drift, it was decided to simply skip ten calendar days; Thursday 4 October 1582 on the Julian calendar was followed by Friday 15 October 1582 on the Gregorian calendar. Parts of the Low Countries (Brabant, Zeeland, the County of Holland, and the States General of the Netherlands) adopted the Gregorian calendar in 1582, while other parts of the Low Countries (Frisia, Overjissel, Utrecht) switched in 1700; Great Britain and its colonies kept using the Julian calendar till 1751. While living in Leiden, the Pilgrims must have used the Gregorian calendar, because others around them did so, but they certainly did not approve of what was seen as papal usurpation of a civil prerogative, namely, the determination of a calendar. William Bradford's journal, Of Plimoth Plantation uses the Julian calendar throughout.
One year is 365.24219 days, so 400 years is 146,096.876 days. If you want to commemorate something 400 years after it happened, you should do so 146,097 days later. That number happens to be a multiple of seven, so you’ll actually end up on the same day of the week [!!], which is a nice bonus. It does not matter in which calendar you add the 146,097 days; if you start with the same day in one calendar, you end up with the same day in that same calendar. Four hundred years after 5 October 1620 on the Julian calendar is 2 October 2020 on the Julian calendar (5 October 2020 minus three days: see below). Four hundred years after 15 October 1620 on the Gregorian calendar is 15 October 2020 on the Gregorian calendar. Back in 1620, the difference between the two calendars was ten days; it has since grown to thirteen days. The date 2 October 2020 [Julian calendar] and 15 October 2020 [Gregorian calendar] are the same day. But Thursday, 15 October 2020 is exactly 400 years since Thursday, 15 October1620.
To make this slightly more confusing, the first date of the new year was 25 March each year, since this was the anniversary of the Incarnation of Christ. Thus, although we start the new year on 1 January, for the Pilgrims, the new year did not begin for almost three months. William Mullins died on 3 March 1621 (Gregorian calendar), but 21 February 1620 in the Julian calendar. The Calendar (New Style) Act 1750 (24 Geo II c.23; also known as Chesterfield's Act after Philip Stanhope, 4th Earl of Chesterfield), an Act of the Parliament of Great Britain, had two parts: first, it reformed the calendar of England and the British Dominions so that the new legal year began on 1 January, rather than 25 March (Lady Day); and, second, Great Britain and its Dominions adopted (in effect) the Gregorian calendar, as already used in most of western Europe.