Detail of Leiden map, ca. 1600, a hand-colored engraving created by Pieter Bast, showing the Pieterskerk and surrounding area. Note the clock tower that gave Clock Alley its name. The boats on the Rapenburg show where the Pilgrims boarded. Courtesy of Erfgoed Leiden en Omstreken (Heritage Leiden and Region)

There are many Mayflower myths already, but the Mayflower 400 year brings new ones. The very latest Mayflower myth is that the Pilgrims boarded the Speedwell in Leiden. The simple truth is that the Speedwell was never in Leiden. The Pilgrims took canal boats to Delfshaven, where the Speedwell was waiting for them, and set sail for Southampton. A widely shared blog post proposes an alternative myth: the Pilgrims travelled from Leiden to Delfshaven on foot, on horseback, and by carriage.

A myth that’s been repeated a lot the last year or so is that the Pilgrims boarded those canal boats at a spot marked by a statue. The text on the base of that statue reads “From here the Pilgrims left Leiden on their journey to the new world,” and that text is easily misunderstood. The statue is near the Vliet Bridge, and the text wouldn’t be misunderstood if the statue had been placed on that bridge instead of merely close to it.

Back in 1620, the bridge was part of the border wall of Leiden, and the Pilgrims left Leiden when they crossed under that bridge. They did not board at that spot. They boarded at the Rapenburg, not far from the Pieterskerk and John Robinson’s house.

A fourth myth that was widely spread in recent days is that the Pilgrims left Leiden on 21 July 1620, and boarded the Speedwell on 22 July 1620. Everything actually happened ten days later. They left Leiden on 31 July 1620 and boarded the Speedwell on 1 August 1620. The reason for this myth is calendar confusion.

Calendar confusion

Confusion about the dates is caused by the fact that the Pilgrims used another calendar than we do. The Pilgrims used the Julian calendar, while we use the Gregorian calendar. It is easy to mistake a Julian date for a Gregorian date or vice versa, as the Julian and Gregorian calendars are nearly identical. The difference between the two is when they have leap days.

The most important thing to understand about the difference between the Julian and Gregorian calendars is that because they are different, you cannot take a date from one calendar and simply use that same date with the other calendar, because what reads and sounds like the same date is actually a different day. Well, technically, the two calendars do “agree” for 1 March 200 CE through 28 February 300 CE, but those dates do not readily occur in Mayflower history.

Julian versus Gregorian

There actually is more than one type of year. Different definitions make for slightly different length. Calendars aim to match the so-called tropical year, which is 365.24219 days long.

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.

Pope Gregorius XIII

The Julian calendar is named after Julius Caesar, who introduced it as a reform of the Roman calendar. The Gregorian calendar is named after Pope Gregorius XIII, who introduced it as a reform of the Julian calendar. The Catholic Church cared about the calendar because they noticed Easter shifting relative to the seasons and really did not like that.

The Gregorian calendar was introduced in 1582 CE. At that time, the accumulated error of the Julian calendar since the Council of Nicaea in 325 CE was ten leap days. The calendar had drifted ten days relative to the seasons. 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.

Adoption

The Catholic Pope was (and is) powerful, and the Gregorian calendar is objectively better than the Julian calendar, but calendars are a civil matter – not decided by churches, but by governments. Adoption of a new calendar requires a government decision.

Many Catholic countries were quick to adopt the Gregorian calendar, but Protestant and Eastern Orthodox countries were not eager to follow any Roman Catholic decree. Adoption of the Gregorian calendar is a long and complex story, but we only care about England and Holland.

The Low Countries adopted the Gregorian calendar in 1582; England did not. Henry VIII had thumbed his nose at the pope by creating the Anglican Church, with the English head of state as the head of the Anglican Church. The English monarch was not going to jump at some papal decision. Great Britain and the many English colonies kept using the Julian calendar till 1751.

The Pilgrims

When the Pilgrims lived in Leiden, the Dutch were using the Gregorian calendar, while the English were still using the Julian calendar. The Pilgrims became Separatists and fled to the Netherlands because they did not like how Catholic the Anglican Church still was. They did not like the Anglican Church, but they liked the Catholic Church even less. While living in Leiden, the Pilgrims must have used the Gregorian calendar, because everyone around them did so, but they tried to stick to their English identity. William Bradford's journal, Of Plimoth Plantation, the main source for the early history of the Pilgrims, uses the Julian calendar throughout.

Julian versus Gregorian

Both calendars have their uses. While it generally makes sense to use Gregorian calendar dates, because that is what everyone else is using nowadays, it also makes sense to use Julian calendar dates when referring to sources that use the Julian calendar. 
What doesn’t make sense is arbitrarily mixing and matching dates from those two calendars, as that will only create confusion. The aforementioned widely shared blog post (which shall remain otherwise unidentified, to protect the guilty) mentions two dates in the first two paragraphs. The blog post claims that the Pilgrims left Delfshaven on 22 July 1620 and left Plymouth on 16 September 1620. Those two dates together are nonsense, as the first date is only right when you’re using the Julian calendar, while the second date is only right when you’re using Gregorian calendar.

The blog post should say that the Pilgrims left Delfshaven on 1 August 1620 and Plymouth on 16 September 1620. It could also say that the Pilgrims left Delfshaven on 22 July 1620 and Plymouth on 6 September 1620, but, if it did, the post should make it clear that these are dates on the Julian calendar.

It is the day that matters

If you’re looking for a general rule, I’d say that it’s best to use the Gregorian calendar until you have a good reason to use the Julian calendar. What constitutes a good reason is a matter of judgment. In a very real sense, it isn’t the calendar that matters, it is the day that matters. You can choose whichever calendar you like, but you still have to identify the right day.

You can say that the Mayflower Pilgrims left Leiden on 21 July 1620 (Julian calendar) or that they left Leiden on 31 July 1620 (Gregorian calendar), just be clear about which calendar you are using, to avoid claiming that they left Leiden on 31 July 1620 (Julian calendar) or 21 July 1620 (Gregorian calendar).

Quadricentennial date

The Mayflower Pilgrims left Leiden on 21 July 1620 of the Julian calendar. Commemorating that on 21 July 2020 of the Gregorian calendar makes no sense. You just cannot mix and match dates and calendars like that.

There are two obvious candidate dates for the quadricentennial. If we were still using the Julian calendar, we would surely commemorate the departure on 21 July 2020 of the Julian calendar. However, because the world did switch to the Gregorian calendar, and the Pilgrims departed on that calendar's 31 July 1620, we commemorate the departure on 31 July 2020. That last date is the right date, and not just because the Dutch were already using the Gregorian calendar. It is the right date because it is exactly 400 years later.

146,097 Days

One year is 365.24219 days, so 400 years is 146,096.876 days. Thus, 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 weekday, which is a nice bonus.

Four hundred Julian calendar years equals 146,100 days, which is three days too many, while 400 Gregorian calendar years equal 146,097 days, which is spot on. If you use the Gregorian calendar, you can find the right date by simply adding 400 to the year, but if you use the Julian calendar, you must also subtract 3 days.

It does not matter in which calendar you add the 146,097 days; if you start with the same day in different calendars, you end up with the same day in different calendars. Four hundred years after 21 July 1620 on the Julian calendar is 18 July 2020 on the Julian calendar (21 July 2020 minus three days). Four hundred years after 31 July 1620 on the Gregorian calendar is 31 July 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 18 July 2020 on the Julian calendar and 31 July 2020 on the Gregorian calendar are the same day. What really matters is that 31 July 2020 is the right day: exactly 400 years since 31 July 1620.