The Feast of Saints Peter and Paul or Solemnity of Saints Peter and Paul is a liturgical feast in honor of the martyrdom in Rome of the apostles Saint Peter and Saint Paul, which is observed on 29 June. The celebration is of ancient origin, the date selected being the anniversary either of their death or of the translation of their relics.
More at Wikipedia.
Image: Saint Peter and Saint Paul. Oil on canvas by El Greco, circa 16th-century. Hermitage Museum, Russia.