The calendar that is used almost universally today is known as the Gregorian calendar.
The Julian calendar was first implemented by Julius Caesar in 46 B.C. It defined a year as having an average length of 365.25 days. In fact, this was 10.8 minutes too long. Over the course of 400 years, this resulted in the calendar being 3 days behind.
In 1582, Pope Gregory XIII trimmed 10 days from the calendar (October 4th 1582 was followed by October 15th 1582). In addition he decreed that from then on all years evenly divisible by four would be leap years, except those that were evenly divisible by 100 and not divisible by 400. The new calendar was named the Gregorian calendar.
Unfortunately, Pope Gregory’s papal edict applied only to lands over which the Catholic Church had dominion. Protestant countries did not adopt it until later. Most protestant countries adopted the calendar between 1699-1701. Great Britain, Ireland and the American colonies, did not switch over until 1752 - by which time it was necessary to cut 11 days from the calendar, instead of 10, to bring the British calendar into line. Wednesday September 2nd 1752 was immediately followed by Thursday September 14th 1752.
After 1752, most major Western countries were using the Gregorian calendar. Some smaller countries took much longer to go over to using it. Romania did not switch to it until 1919. Turkey did not use it until 1927.
The Gregorian calendar made another change too. It set the starting date of the year as January 1st. Previously, individual countries celebrated New Year on different days. Before 1752, England and the American colonies used March 25th as New Year’s Day. For example, March 24 1719 was followed by March 25 1720. One consequence of these changes is that references to dates prior to the switchover can be ambiguous. Does the year refer to the date as it was specified then, or as we would specify it now? To remove these ambiguities a system called 'double dating' is often used (see Simple Date Fields).
As we have seen, the day that in England would have been recorded as Wednesday September 2nd 1752, would have been Wednesday September 13th 1752 in most of continental Europe. Therefore it is not enough to simply have a date. To know what the date means, you have to know where the date was used.
When displaying dates, you can sometimes opt to display the day-of-the-week associated with that date - e.g. in a Query result set. But, the day of the week - at least for dates between 4th October 1582 and September 14th 1752 - will depend on when the Gregorian calendar is deemed to have been introduced. This ‘changeover date’ also has an effect on the validation of dates in the same period. Was 1700 a leap year or not, for example?
For the purposes of calculating days-of-the-week and validating dates (between 1582 and 1752), Family Historian allows you to choose either the 1582 changeover date, or the 1752 changeover date (see Preferences). If your dates are wholly or mainly for Great Britain, Ireland and/or America, you may wish to use 1752. However, this is not a key decision. You can switch back and forward between 1752 and 1582 whenever you wish to. Doing so will not change the data in your file in any way. Nor does it matter if your file contains dates, which would not be valid by one reckoning or the other (or even both!). Family Historian allows you to store invalid dates in any case. Apart from affecting the way that days-of-the-week are calculated, the only other practical effect of changing the ‘changeover date’ setting is that it will affect which warnings you may get when you enter dates. However, you can always ignore these warnings and keep the date anyway.