Pentecostarion
During Bright Week (the week beginning on Easter Sunday) the services are completely different from the rest of the year, and there is no usual beginning.
The hymn, "O Heavenly King..." (above) is one of the propers of Pentecost, and will not be said from Pascha (Easter) until the feast of Pentecost, fifty days later.
From Thomas Sunday until the Great Feast of the Ascension, "O Heavenly King" is replaced with the Troparion of Pascha:
Christ is risen from the dead, trampling down death by death, and upon those in the tombs bestowing life! (Three times)
From Ascension until its leavetaking, "O Heavenly King" is omitted entirely, and replaced by nothing (symbolizing the physical "absence" of Jesus and the expectant coming of the Holy Spirit). Instead, immediately after the priest's blessing, the reader says: "Amen. Holy God..."
Read more about this topic: Usual Beginning