Firstly Jesus was born before 4 BC (when Heriod died) using the current dating system so the celebrations are for his nominal birth date at the end of 1 BC and the start 1 AD. Note that there is no year zero - BC and AD are measured using ordinals (first, second etc) and not cardinals (one, two etc.). So 2000AD represents the two thousandth year which will not be completed until the end of the year.
The date of the birth of Christ was calculated by a monk called Dionysius Exiguus in 525 AD. He assumed that the Annunciation (when the angel Gabriel appeared to Mary) was on the spring equinox in the year 752 AUC (ab urbe condita or the foundation of Rome) and used this as the basis of his dating system (anno ab Incarnatione). The spring equinox was on 25th March with the year beginning at the start of that month (hence the names of September through to December or 7th to 10th months). The birth would be 9 months later on midwinter day 25th December (there were only two seasons, Winter and Summer, which started and finished on the equinox and hence midsummer's day is June 24th). Thus the birth date was 25th December 1 BC - the start of 1AD (modern) was 8 days later - day of the naming and circumcision. (Using January 1st as the start of the year is still called stilo circumcisionis).
The English calendar was established by the Venerable Bede at the start of the eighth century. He used the Dionysius year count, but with the autumn equinox starting the year.
This system was used until the 10th century when the start of the year was changed back to St Mary's feast day (25th March). However Bede had added 6 months in moving the start of the year to September, and another 6 months were added in moving it back to March. The original Dionysius dates are measured from 25 March 1 BC (known as the Pisean style) while the English dates are measured from 26 March 1 AD (Florentine style) - one year after the Annunciation. In 1752 the English New Year was moved back to January 1st (thus 1751 had only 282 days) which had been used throughout the rest of Europe for two centuries (Scotland changed in 1601 so Elizabeth 1st died in 1602 in England and 1603 in Scotland). Prior to 1752, if a couple married in late March, April or May their first child could be born in January, February or early March of the same year.
The Gregorian reform of the calendar was implemented from 1st January 1582 in most of western Europe (by missing 10 days in October) which became the start of the new year, but not in the British Isles. Pope Gregory XIII corrected the Julian year of 365 days and 6 hours by 11 minutes and 14 seconds by skipping the leap year if the year was divisible by 100 unless it was divisible by 400. England changed to the Gregorian calendar in 1752 deleting the 11 days from September between the 2nd and 14th (1752 had 355 days). Thus Christmas should have occured 11 days later (January 6th also known as Old Christmas Day), but the date of December 25th was retained. The date of the equinox and solstice now occur on the 21st or 22nd (depending on the leap year cycle), but midsummer's day remains on the 24th June (Midsummer is one of the four quarterdays - and Lady Day, Midsummer, Michaelmas, and Christmas - see below). Feast days were not adjusted, but fairs were so that fairs that had occured on feast days were now 11 days later. The start of the financial year which had coincided with the calendar year now starts 11 days later on April 6th. Note that the different countries in Europe have moved to the Gregorian calendar at different times. The Eastern Orthodox churches still use the Julian Calendar where every fourth year is a leap year and so are now 13 days behind the Gregorian calendar. The oriental Orthodox churches use a modified Gregorian calendar where century years are only leap years if dividing by 9 leaves a remainder of 2 or 6.
The rural economy used the quarter-days until very recently.
25 March, Lady Day, was the beginning of the year until 1752, at least for the Established Church and for legal matters. Laborers who were hired at fairs held in February, often at the time of Candlemas (2 February), began their year of work at Lady Day. Those who paid rent half-yearly paid at Lady Day and at Michaelmas; the Hearth Tax of the late 1600s was collected at Lady Day and at Michaelmas.
24 June, Midsummer, was probably celebrated in English parishes by a church-ale, a fund raising event for the parish so named because good, stout ale was supplied for the enjoyment of all.
29 September, Michaelmas, or the Feast of St. Michael and All Angels in the Church of England. Rents were due and taxes collected.
25 December, Christmas Day, a break before the round of work began again, for tillage started at Candlemas.
The four term days in Scotland were Candlemas, Whitsunday, Lammas Day, and Martinmas. In 1886, the term days for the beginning and ending of employment of servants in towns were set as 28 February, 28 May, 28 August, and 29 November.
2 February, Candlemas, which fell forty days after Christmas, marked the presentation of the infant Jesus in the temple and the purification of the Virgin Mary. Though not a quarter day in England, it was when important agricultural fairs were held.
Whitsunday, originally a moveable term day, coming the seventh Sunday after Easter, was fixed in Scotland at 15 May in 1693. Whitsunday was originally the feast of Pentecost, around which a great many christenings would occur, so it became associated with the color white. Lammas Day Lammas Day, 1 August, feast of St. Peter ad Vincula was a corruption of loaf-mass, the Sunday on which the first fruits of harvest were offered, first corn ground, and first loaf made. In Scotland it was associated with hand-fasting and some fairs on this day were called handfasting fairs. (Originally synonymous with betrothal, handfasting became a contract binding a man and woman to live together for a year and a day before they decided on permanent marriage.)
Martinmas, 11 November, was known as St. Martin in Winter or St. Martin of Tours to distinguish this from another feast of St. Martin in July.