This day is honoured in the Liturgy by what is called Saint Mark’s Procession. The term, however,is not a correct one, inasmuch as a Procession was a privilege peculiar to the twenty-fifth of April previously to the institution of our Evangelist’s feast, which, even so late as the sixth century, had no fixed day in the Roman Church. The real name of this Procession is The Greater Litanies. The word Litany means Supplication, and is applied to the religious rite of singing certain chants while proceeding from place to place, and this in order to propitiate Heaven. The two Greek words Kyrie eleison (Lord have mercy on us) were also called Litany, as likewise were the invocations which were afterwards added to that cry for mercy, and which now form a Liturgical prayer used by the Church on certain solemn occasions.
The Greater Litanies (or Processions) are so called to distinguish them from the Minor Litanies, that is, Processions of less importance as far as the solemnity and concourse of the faithful were concerned. We gather from an expression of Saint Gregory the Great that it was an ancient custom in the Roman Church to celebrate, once each year, a Greater Litany at which all the Clergy and people assisted. This holy Pontiff chose the twenty-fifth of April as the fixed day for this Procession and appointed the Basilica of Saint Peter as the Station.