13 Wherefore, We decree and command that throughout the whole Catholic Church,this year and in every subsequent year, a Novena shall take place before Whit-Sunday, in all parish churches, and also, if the local Ordinaries think fit, in other churches and oratories. To all who take part in this Novena and duly pray for Our intention, We grant for each day an Indulgence of seven years and seven quarantines; moreover, a Plenary Indulgence on any one of the days of the Novena, or on Whit-Sunday itself, or on any day during the Octave; provided they shall have received the Sacraments of Penance and the Holy Eucharist, and devoutly prayed for Our intention. We will that those who are legitimately prevented from attending the Novena, or who are in places where the devotions cannot, in the judgment of the Ordinary, be conveniently carried out in church, shall equally enjoy the same benefits, provided they make the Novena privately and observe the other conditions. Moreover We are pleased to grant, in perpetuity, from the Treasury of the Church, that whosoever, daily during the Octave of Pentecost up to Trinity Sunday inclusive, offer again publicly or privately any prayers, according to their devotion, to the Holy Ghost, and satisfy the above conditions, shall a second time gain each of the same Indulgences. All these Indulgences We also permit to be applied to the suffrage of the souls in Purgatory.
Source: Divinum Illud Munus (Vatican.va)