In the Eastern Orthodox churches which still follow the Julian calendar, the Nativity Fast is celebrated November 28 to January 6. The Eastern Fast runs for 40 days and thematically focuses on the proclamation and glorification of the Incarnation of God.

During the Nativity Fast the Church encourages us to intensify our spiritual lives in order to ready our souls and bodies for the Feast. We do this through fasting, prayer, alms-giving, confession and increased attention given to "preparing for His coming", to make sure we've done what we can to make a "fit dwelling place for Him" in our hearts and lives. It is believed that by tempering the bodily desire for food, other passions are tempered as well, and that the soul can orient more away from worldly needs and more towards spiritual needs.

Nativity Fasting Rules may vary in different jurisdictions, however, here are the basic rules.

The fast traditionally entails fasting from red meat, poultry, meat products, eggs, dairy products, fish, oil, and wine. During the early part of the fast, the rule is identical to that of the Apostles' Fast (see below). During the latter part of the fast, fish is no longer eaten on Saturdays or Sundays. In different traditions, this heightening of the fast may be for either the last week or the last two weeks.

Apostles’ Fast:

The rule for this fast is more lenient than for Great Lent.

Monday, Wednesday, Friday: Strict fast and no consuming food until the evening sunset.

Tuesday, Thursday: Oil and wine permitted.

Saturday, Sunday: Fish, oil and wine permitted.

There are many exceptions to the broad rules given here, such as when a major feast day, or the patronal feast of a parish, falls during a fasting period.

Each individual is expected to confer with their confessor regarding any exemptions from the fasting rules, but should never place themselves in physical danger.