26 To achieve greater unanimity in this prayer, Venerable Brethren, We would have you organize a solemn novena to the Holy Spirit in all the parishes of your diocese immediately preceding the Ecumenical Council. The object of this novena will be to beg for an abundance of heavenly light and supernatural aid for the Fathers in council. To all who join in this novena We impart from the Church's treasury a plenary indulgence, obtainable on the usual conditions.
Source: Paenitentiam Agere (Vatican.va)