Following Pentecost, there are several moveable feasts that occur each year, such as Corpus Christi, Trinity Sunday and the Solemnity of the Sacred Heart.
In recent decades, several bishops requested another feast to be celebrated on the Thursday following Pentecost. Initial approval of the feast for specific dioceses was given in 1987.
The feast is named in honor of "Our Lord Jesus Christ, the Eternal High Priest." It is celebrated locally in England, Wales, Poland, Slovakia, the Netherlands and Spain.
The Liturgy Office for England and Wales has all the liturgical texts posted on its website, along with a letter authorizing this feast in 2013:
At the request of His Eminence Vincent G. Nichols, Archbishop of Westminster, President of the Bishops’ Conference of England and Wales, made in a letter dated 11 December 2012, in virtue of the faculties granted to this Congregation by the Supreme Pontiff Benedict XVI, we are delighted to grant that the feast of Our Lord Jesus Christ, the Eternal High Priest be inserted into the proper Calendar of England and the proper Calendar of Wales with the rank of feast on the Thursday after Pentecost.
The feast emphasizes Jesus' role as priest, offering himself as a sacrifice for all.
Jesus the Eternal High Priest
Pope Pius XII wrote about this aspect of Jesus' ministry in his encyclical, Mediator Dei:
Christ is a Priest indeed; however, he is a Priest not for himself but for us, since, in the name of the whole human race, he brings our prayers and religious dispositions to the eternal Father; he is also a victim, but a victim for us, since he substitutes himself for sinners.
While this feast is only celebrated in certain parts of the world, there does exist in the current Roman Missal a votive Mass of Our Lord Jesus Christ, the Eternal High Priest, which can be celebrated during Ordinary Time when there is not an obligatory memorial on a specific day.
The collect for the Mass provides for us another summary of the spiritual theme behind this celebration:
O God, who for your glory and the salvation of the human race willed to establish Christ as the eternal High Priest, grant that the people he has gained for you by his Blood may, through their participation in his memorial, experience the power of his Cross and Resurrection. Who lives and reigns with you in the unity of the Holy Spirit, God, for ever and ever.
While this feast isn't on the universal calendar, we can still participate in it if we feel drawn to honor Jesus' role as Eternal High Priest.
