The Apostles' Fast follows the joy of Pentecost, beginning the Monday after All Saints and ending on June 28, the eve of the feast of the holy apostles Peter and Paul. Having received the Holy Spirit, the apostles went out to preach, fasting and praying as they went; the Church keeps this fast in their company, and in preparation for their feast.
Its length varies from year to year, a consequence of the moving date of Pascha, from which Pentecost is reckoned. Some years it is several weeks; some years, on the New Calendar, it is very short or falls away entirely. This variability is itself a small teaching: the fasts serve the feasts, and bend to the paschal rhythm that governs the whole year.
Milder than the great fasts, the Apostles' Fast is nonetheless a real one, kept as the Church appoints and as one's priest directs. It sends the summer, too, toward God, and honours those who first carried the Gospel to the world.