The two leading saints of the early Church are celebrated together in the Feast of Saints Peter and Paul, (or the Solemnity of Saints Peter and Paul if celebrated on a Sunday). This celebration is a liturgical feast in honor of the martyrdom in Rome of the apostles Saint Peter and Saint Paul and is observed on June 29th.
The Roman Martyrology (the official lists of solemnities, feasts, and other liturgical celebrations of the saints and martyrs) assigns June 29th as the feast day without thereby declaring that to be the day of their deaths. St. Augustine of Hippo (late 4th century) says in his Sermon 295: “One day is assigned for the celebration of the martyrdom of the two apostles.
But those two were one. Although their martyrdom occurred on different days, they were one.” Thus it is clear that the celebration is of ancient origin. There is some speculation that the date selected may be the anniversary either of their death or of the translation of their relics.