All Saints' Sunday (Greek: Αγίων Πάντων), the first Sunday after Pentecost (Whitsun), is dedicated to all the Saints, as the "harvest of the Spirit of God" and the "fruits of Divine grace" sent to people on the day of Pentecost. Since not all canonized Saints have their days designated for celebration in the calendar, and especially since the names of all those glorified by the Lord are not known, the Church established this holiday.
Even in the time of Saint John Chrysostom, the memory of all the Holy Martyrs was celebrated on the Sunday after Pentecost. From a poem by the Venerable Ephraim the Syrian, it is seen that the memory of All Saints was celebrated on May 13, and from a letter of Emperor Leo VI the Wise (886-912), it is seen that the memory of All Saints was celebrated after the Sunday of Pentecost. In the service of this day and the synaxarion, all the Saints are listed in order of rank: forefathers, patriarchs, prophets, apostles, martyrs, priests, martyrs, holy martyrs, venerable, righteous, and the entire cast of holy women.
This feast marks the end of the Palm Triodion and the beginning of the ordinary season of the year, when worship is performed according to the Octave and Menaeus. From the Monday after All Saints' Sunday, the Petrine or Apostolic Lent begins, which serves as a preparation for the great feast in honor of the Holy and First-Supreme Apostles Peter and Paul on June 29.
After Pentecost, all Sundays, that is, all days of the week, are designated by ordinal numbers, starting with the first (All Saints' Sunday), and ending with the thirty-second. In accordance with this, the columns of the Easter morning Gospels and the columns of the voices of the Octave are also counted. The column of the Gospels begins on All Saints' Sunday, when the first morning Gospel of Easter is read at Matins. But the column of the voices of the Eight-voiced choir begins the following, second Sunday. For that Sunday, the first voice is assigned, and the second Gospel. From then on, everything goes in order. Starting on All Saints' Sunday, the katavasia is "I will open my mouth..."
Source: Diocese of Šumadija