The Calendar (New Style) Act 1750 (24 Geo. 2. c. 23), also known as Chesterfield's Act or (in American usage) the British Calendar Act of 1751, is an act of the Parliament of Great Britain. Its purpose was for Great Britain and the British Empire to adopt the Gregorian calendar (in effect). The act also changed the start of the legal year from 25 March to 1 January.
The act elided eleven days from September 1752. It ordered that religious feast days be held on their traditional dates – for example, Christmas Day remained on 25 December. (Easter is a moveable feast: the act specifies how its date should be calculated.) It ordered that civil and market days – for example the quarter days on which rent was due, salaries paid and new labour contracts agreed – be moved forward in the calendar by eleven days so that no-one should gain or lose by the change and that markets match the agricultural season. It is for this reason that the UK personal tax year ends on 5 April, being eleven days on from the original quarter-day of 25 March (Lady Day).
The act has three key elements:
It acknowledges the practical difficulties created for England by its traditional practice of beginning the year on 25 March, when common practice among its people, as well as legal and common usage in Scotland (since 1600) and most of Europe, was to begin the year on 1 January. Accordingly, the act provided that "all His Majesty's Dominions" would cease this tradition from the end of December 1751, such that the following day would be 1 January 1752.
It acknowledges that the Julian calendar still in use in Great Britain and its Dominions had been found to be inaccurate, and that most of Europe had already adopted an (unnamed) revised calendar. The Julian calendar was eleven days behind this 'New Style' calendar. With this Act, therefore, Britain implicitly adopted the Gregorian calendar. To do so, it ordered that eleven calendar days be skipped in September 1752 and that centennial years no longer be leap years unless divisible by 400.
An annexe to the act gives a method to calculate the dates of Easter by varying slightly the traditional method of the Church of England (rather than by adopting the method of Pope Gregory XIII).
The act also provides that fixed religious feast days continue to be observed on the same calendar date, whereas movable feasts (whose dates depend on the date of Easter) would follow from the new rules for its calculation. Religious feast days were to be held to their nominal dates (for example Michaelmas, on 29 September), but the act also requires that the dates of 'fairs and marts' traditionally associated with those feasts (but in reality tied to the seasons), be moved in the calendar by discounting eleven days. Thus, for example, the Michaelmas hiring and mop fairs moved to 10 October and became known as Old Michaelmas Day. Christmas Day was still celebrated on 25 December and an 'Old Christmas' was not formalised: nevertheless, some communities were reluctant to accept the change.
The act includes various measures to prevent injustice and other problems. For example, Section VI, echoing a rule in Gregory's reform, provides that the date on which rents and other debts are due must be deferred by 11 days. In addition, the same Section says a person does not reach a particular age, including especially the age of majority (21), until the complete number of years have passed.
In addition, the act finally settles the position of leap day in English law as 29 February.
By the 18th century, the English legal year – used for legal, financial and other civil purposes – had for centuries begun on 25 March, or Lady Day. Thus, for example, 24 March 1707 was immediately followed by 25 March 1708, while the day following 31 December 1708 was 1 January 1708, with 1709 still nearly three months away. The introduction to the act states succinctly the rationale for a change to the start of the year in England (and Wales) to 1 January: the March date had been found to have many inconveniences. It differed from the date (1 January) used by Scotland, which had ceased to use 25 March in 1600, as well as that used by other neighbouring countries and ordinary people throughout the kingdom. As a consequence, it says, frequent mistakes were made in the dates of deeds and other writings.
In The Twelve Caesars, Suetonius wrote that the Julian calendar, introduced by Julius Caesar in 45 BC as consul, continued the old Roman practice of beginning the year with 1 January. Later Christians, however, felt that 1 January had no religious significance and wanted to begin the year on a more appropriate date, resulting in a great variety of dates being used during the medieval period. No civil legislation or religious canon law ordered this change, but Christmas Day on 25 December gradually became a popular date for New Year in England from the 6th century, though this custom fell almost entirely out of use by the beginning of the 14th century. It is not wholly clear why 25 March came to be settled on as the beginning of the legal year instead, though it saw use in Aquitaine during the 10th century and Normandy by the 11th (shortly before the Norman Conquest). The 11th century also saw dedications to the Virgin Mary go from being rare in England to the most frequent, signifying the increasing importance of Lady Day in the liturgical year. Despite being commonplace by the middle of the 12th century, however, it then gave way to the invariable use of the regnal year for legal records from the reign of Henry II, though its legal status was cemented by its use in official parliamentary records from the 16th century onwards. From the 13th century, there also arose the curious phenomenon of New Year's Day being almost universally celebrated on 1 January, despite the recorded year not incrementing until 25 March.
However, it was the continuation of the Roman calendar layout beginning with January that eventually led European countries in the 16th century to return to a legal year that started on 1 January: for example the Venetian Republic (1522, sixty years before the Gregorian reform), France (1564) and Scotland (1600). By 1750 most of Europe had already made this change and the continuing English practice became a source of confusion for English merchants and diplomats and their counterparts, when dealing across the Channel or with Scotland. An informal system of 'dual dating' had developed to help reduce confusion. For example, a date written as 21 January 1719/20 (or 17+19/20) means both a date of 21 January 1719 (where the year began ten months earlier, on 25 March 1719, as in England) and a date of 21 January 1720 (where the year began three weeks earlier, on 1 January 1720, as in Scotland). These notations both refer to the same day in the real world. The Calendar (New Style) Act 1750 removed this difficulty by changing the start of the year to 1 January for England and Wales and the colonies. The change applied "after the last day of December 1751". The legal year which began on 25 March 1751 became a short year of 282 days, running from 25 March 1751 to 31 December 1751. The following year began as 1 January 1752 (New Style).
The reason given to discard both the traditional calendar and the eleven-day accumulated difference was a religious one: calculation of the date of Easter. The introduction to the act explains that, due to the inaccuracy in the Julian calendar, the date of the spring equinox (which determines the date of Easter) had drifted by about eleven days from its date at the time of the First Council of Nicaea, 21 March, and that this drift would continue unless the calendar was corrected and the eleven days' difference deleted.
The Gregorian calendar was a reform of the Julian calendar, instituted by Pope Gregory XIII in 1582 by the papal bull "Inter gravissimas" ("Among the most serious"). The intention expressed by the text of this bull was to reset the calendar so that celestial events critical for the calculation of Easter dates – the March equinox and its adjacent full moons – would be back in what the bull calls "their proper places" and would be prevented from being moved away again.