The Nativity Fast also called the Philip's Fast
In brief
The Nativity Fast is the forty-day period of preparation before the feast of Christ's Nativity — a kind of Advent, though older and quieter than the Western season of the same name. It runs from November 15 to December 24 and is milder than Great Lent, with fish allowed on many days early on and the strictness rising as Christmas nears. Like every Orthodox fast, it is joined to prayer and almsgiving and adjusted to the person.
Forty days before the feast
The Church does not arrive at its great feasts unprepared. Just as Great Lent readies the faithful for Pascha, the Nativity Fast readies them for the Nativity of Christ on December 25 — a smaller, gentler "little Lent" of forty days. It begins on November 15 and ends on the eve of the feast, December 24. It is often called the Philip's Fast because it opens on the day after the feast of the Apostle Philip (November 14), an easy landmark by which to remember the start.
The fast is ancient, though it took its settled forty-day shape gradually over the first Christian millennium. Its purpose is not gloom but expectation: to keep the weeks before Christmas as a season of watching and hunger, so that the feast, when it comes, lands on a soul made ready. The prophecies read at the services set the tone — "For unto us a child is born, unto us a son is given" (Isaiah 9:6); "But thou, Bethlehem Ephratah... out of thee shall he come forth" (Micah 5:2) — the Church leaning forward toward the mystery that "when the fulness of the time was come, God sent forth his Son" (Galatians 4:4).
How the strictness varies
The Nativity Fast is notably less severe than Great Lent, and it is kept in two phases. In the common calendar of many jurisdictions, from November 15 through December 19 the fast follows the ordinary rule — no meat, eggs, or dairy — but fish, wine, and oil are permitted on Saturdays and Sundays, and on feast days such as the Entrance of the Theotokos (November 21). From December 20 through the eve on the 24th the fast tightens: fish is no longer eaten even on the weekend, as the Church draws its breath for the feast. The final day, the Eve of the Nativity, is kept strictly.
These particulars vary by jurisdiction and by household — Greek, Slavic, and Antiochian calendars differ in their details, and a person's own priest may relax or adjust the rule through economia for health, age, travel, or circumstance. No newcomer is expected to keep the full rule at once. The honest counsel is to fast as one is able, with a blessing, and to remember that the calendar is a servant of the soul, not a scoreboard.
A fast of glad expectation
The Nativity Fast runs against the grain of the surrounding culture, where the weeks before Christmas are the loudest and most indulgent of the year. The Church spends them the other way: eating simply, giving to the poor, and keeping the parties for the twelve days of feasting that begin — not end — on Christmas. Landmarks brighten the road, above all St. Nicholas on December 6, whose generosity sets the season's key. Kept this way, the fast does for Christmas what an empty stomach does for a feast: it makes one truly hungry for the thing itself, so that the newborn Christ arrives to a people actually waiting for Him. For the reasons behind fasting in general, see Why Orthodox Christians Fast.