Change from Julian to Gregorian Calendar
Before describing the changes in the calendar that took place in 1751/52, here are brief descriptions of the calendars that were in use before that time, the reasons for the changes from one to the next and the problems that the change of 1752 resolved.
The Roman Calendar
The Roman calendar was very complex and the earliest version consisted of ten months starting at the vernal (spring) equinox. However, this version had about 61 days in the winter that did not fall inside the calendar !
Since March was considered the first month of the year, October was so called because it is the eighth month (hence the prefix "oct", as in octagon) and December is the tenth month (hence "dec"). Two months were subsequently added at the end of the year in about 713BC to give a year with 355 days.
In order to keep the year aligned with the solar year a leap month was added from time to time. This is where the complication came from because the addition was decided by an official, not by a regular system, and he would add or not add to suit his own political agenda. The average Roman citizen, particularly if they were remote from the city, did not necessarily know the date and the situation got gradually worse such that the last years before the reforms carried out by Julius Caesar were known as "years of confusion".
The Julian Calendar
Following the advice of astronomers and mathematicians, Julius Caesar established a new calendar to replace the old Roman one in 45 BC and this calendar is known as the Julian calendar. It had a four year cycle consisting of three common years containing 365 days, and one year (leap year) containing 366 days. This twelve-month calendar, based on a solar year, lasted for well over a thousand years in a continuous cycle without any changes.
The Romans named their years after the consuls in power and since they took office on 1st January that is when the name for their new year began. However, in most Western European countries the first day of the civil year was March 25th (often known as Lady Day, Annunciation Day, or Feast of the Annunciation), and the last day of the year was March 24th. Before 1752 in England 1st January was celebrated as the New Year Festival whilst the civil year still started on Lady Day. (see Pepys Diary, for instance)
The Gregorian Calendar
During the Middle Ages, astronomers observed that the calendar year was not completely coincident with matching solar years and was drifting further because the Julian calendar year was slightly too long. Errors were also noted by church officials and scholars because church holidays did not occur in their appropriate seasons. The main problem for the church was that the vernal equinox, used to calculate the date of Easter, had moved slowly forward.
In 1582, Pope Gregory XIII took the advice of Aloysius Lilius and Christopher Clavius and decreed a new, reformed, calendar known as the Gregorian calendar. Although keeping the basic Julian cycle of three years plus a leap year, Gregory also added the more accurate refinement that the leap year be discarded every one hundred years unless the year could be divided by four hundred. This is why the year 1900 was not a leap year but the year 2000 was.
In order to make the calendar adjustment in 1582, ten days were eliminated from October, thus 4th October 1582 was followed by 15th October 1582.
Although only four Roman Catholic countries adopted the calendar at the decreed time the rest followed shortly afterwards. The Protestant countries began adopting it during the eighteenth century onwards, although Sweden was a little indecisive and swayed between the two calendars in turn between 1700 and 1753. They even introduced the unique day of 30th February 1712 in order to revert to the full Julian half way through the change ! The last country to change was Greece in 1923.
Scotland adopted the reformed calendar in 1600, celebrating the New Year on 1st January 1600 and subsequently on January 1st of each year. However, England and it's colonies did not adopt the change until 1752.
England's Adoption of the calendar
When England eventually decided to make the calendar adjustment, eleven days had to be dropped from the month of September 1752. An eleven-day adjustment in 1752 was needed because one more day had been gained since the calendar was changed in 1582. Thus, 2nd September 1752 was followed by 14th September 1752 and September 1752 had only nineteen days.
However, the year 1751 had begun on 25th March and ended on 31st December 1751, meaning that 1751 only had 282 days. At the time of the September 1752 adjustment the first day of the year had already been changed to January 1st and the last day was December 31st i.e. the calendar in use today.
Summary of the 1751/52 Calendar Change :
Note that the effect of the calendar changes didn't happen all at once, but occurred as a series of changes over a period of time.
Because the changes to the calendar took place at different times in different countries, and this resulted in different year numbers for the first three months, documents between 1582 and 1752 often have double dating or use the notation "OS" (Old Style) or "NS" (New Style) to show which calendar is being used.
An example of double dating is:
As a matter of interest, accountants, of course, could not handle the shortening of time.
Lady Day (the original start of the year) was the traditional date for the settlement of accounts and continued as such through the changes. When the calendar was changed in September 1752 Lady Day moved to 5th April and it subsequently moved to 6th April when another Julian leap day was lost in 1800; fortunately the lost leap day in 1900 was ignored.
This is why the UK tax authorities, to the bewilderment of everybody else, use 6th April as the start of the tax year.