Julian Calendar

The Julian Calendar is named after the Roman Emperor Julius Caesar, who introduced it in 46 BCE to link the days of the year (a human construct) to the seasons of the solar year (a natural phenomenon). Before the introduction of the Julian Calendar, Roman dates were calculated according to the Roman calendar (months); Julius introduced essentially the modern system of numbering each day, in order, within each month.

He also, in consultation with a Greek astronomer called Sosigenes, attempted to align the calendar with the solar year by introducing the system of months that is essentially our own, with months of 30 and 31 days, and February having 28 days except every fourth, or leap year, having 29 days.

The Julian Calendar was replaced by the Gregorian Calendar introduced, in Roman Catholic countries by Pope Gregory XIII in 1582, and adopted in the UK not until 1752. Great Britain, like other Protestant countries, was reluctant to adopt a reform instituted by a Roman Catholic - particularly one who still claimed spiritual sovereignty over western Europe.

Years dated according to the Julian Calendar, during the period when some countries used the Gregorian calendar while others (including the UK), are often marked by historians (and by contemporary writes during the period) as O.S. (for Old Style), as opposed to the N.S (New Style) of the Gregorian calendar. N.b. The legal year in Britain under the Julian calendar began on 25 March (Lady Day - the Feast of the Annunciation). When this was adjusted to match the calendar to the solar year, this date was rendered as 6th April, which is still the start of the British tax year, varied to take Sundays into account.