Each page of the calendar shows one winter: the Nativity Fast and Advent of the chosen year and the feasts up to the middle of the following January.
Why 25 December
The Gospels do not give the day of Christ’s birth. The feast on 25 December first appears in a Roman calendar of the mid-fourth century, and by the end of that century it was kept in Constantinople and Antioch as well. Before then, Eastern Christians remembered the Nativity together with the Baptism of Christ on 6 January, the feast of Epiphany. The Armenian Apostolic Church still keeps them together.
Why 7 January
The Russian, Serbian, Georgian and Jerusalem Orthodox Churches, and Mount Athos, follow the Julian calendar, so for them Christmas is 25 December Old Style. The Julian calendar runs behind the Gregorian, civil one: from 1900 to 2100 the gap is 13 days, which puts 25 December Old Style on 7 January New Style.
The gap grows by a day in every century whose number is not divisible by 400: the Gregorian calendar skips the leap day in 1700, 1800, 1900 and 2100, and the Julian calendar does not. In the nineteenth century Orthodox Christmas fell on 6 January, and from 2101 it will fall on 8 January. That is why the calendar covers the winters from 1900 to 2099: in them all Orthodox dates stay the same.
The Churches of Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania and Bulgaria moved to the Revised Julian calendar in the twentieth century and keep Christmas on 25 December together with Western Christians; the Orthodox Church of Ukraine has done so since 2023. All of them still reckon Easter by the Julian computus.
The Nativity Fast and Advent
Orthodox Christians prepare for Christmas with a forty-day fast, from 15 November Old Style (28 November New Style) until Christmas Eve. It begins the day after the feast of the Apostle Philip, which is why it is also called Philip’s Fast.
Western Christians prepare for Christmas during Advent. Its first Sunday is the one nearest to 30 November, the feast of Saint Andrew, that is, a Sunday between 27 November and 3 December; for Catholics and many Protestants it also begins the church year. The fourth Sunday is the last one before Christmas, so Advent lasts from three weeks and a day to four full weeks, when 25 December falls on a Sunday.
Catholics today have no obligatory fast before Christmas: Advent is a season of penance and expectation, kept in violet vestments, but fasting is not prescribed. It once was: from the fifth and sixth centuries Western Christians fasted from the feast of Saint Martin on 11 November — hence St Martin’s Lent — and the period later shrank to Advent. The last obligatory fast days, Christmas Eve and the December Ember Days, were abolished by Pope Paul VI in 1966. Eastern Catholics, including Ukrainian Greek Catholics, keep the forty-day Nativity Fast as the Orthodox do. Most Protestants have no fast before Christmas.
Christmas Eve and Epiphany
Christmas Eve falls on 24 December for Western Christians and on 6 January for the Orthodox. In Slavic languages it is named after sochivo, a lenten dish of soaked grain eaten on that day.
Western Christians celebrate Epiphany on 6 January and remember the visit of the Magi. For the Orthodox it is the Baptism of the Lord, or Theophany — 6 January Old Style, 19 January New Style. In the West the days between Christmas and Epiphany are the Twelve Days of Christmas; in the Slavic Orthodox tradition they are Svyatki, the holy days.