The days within the Octave of Corpus Christi are of semi-double rite.
Everything is sung as on the feast except the antiphons are not doubled and the lessons at Mattins. In the first nocturn the lessons are from the First Book of the Kings. In the second nocturn sermons from SS Chrysostom, Cyprian and Ambrose are read on the Monday, Tuesday and Wednesday respectively. In the third nocturn the homilies on Monday and Tuesday are from St. Augustine and on Wednesday from St. Hilary. Commemorations are sung at Lauds of any occurring feasts.
At Mass Cibavit is sung. The Gloria is sung, the second collect is Concede nos, the third collect Ecclesiae for the Church or Deus omnium. (When a double feast is simplified and is commemorated at Lauds then these prayers are omitted and the feast commemorated. Semi-doubles and simples replace the collects of the season at Mass.) The Sequence Lauda Sion may be sung. The Creed is sung and the preface is of the Nativity.
Vespers are as on the feast, except that the antiphons are not doubled.
A common practice in former times was for both Mass and Vespers to be celebrated coram Sanctissimo, with the Sacrament exposed for both.