Reading from the Synaxarion:

Sunday of the 7th Ecumenical Council

On the Sunday that falls on or immediately after the eleventh of this month, we chant the Service to the 350 holy Fathers of the Seventh Ecumenical Council, which gathered in Nicaea in 787 under the holy Patriarch Tarasius and during the reign of the Empress Irene and her son, Constantine Porphyrogenitus, to refute the Iconoclast heresy, which had received imperial support beginning with the Edict issued in 726 by Emperor Leo the Isaurian. Many of the holy Fathers who condemned Iconoclasm at this holy Council later died as Confessors and Martyrs for the holy Icons during the second assult of Iconoclasm in the ninth century, especially during the reigns of Leo the Armenian and Theophilus.

Apolytikion of Sun. of the 7th Ecumenical Council in the Plagal of the Fourth Tone

You are greatly glorified, O Christ our God, who established our Fathers as luminaries upon the earth, and through them led us all to the true Faith. O Most compassionate, glory to You.

Kontakion of Sun. of the 7th Ecumenical Council in the Plagal of the Fourth Tone

The preaching of the Apostles * and the decisions of the Fathers * have established the true faith of the Church * which she wears as the garment of truth * fashioned from the theology on high. * She justly governs and glorifies the great mystery of worship.