As a rule, whenever St. Paul has a feast day, St. Peter is, likewise, commemorated. Later in the year, on June 29, we will again remember these two apostles with the Feast of the Saints Peter and Paul. Just as they were joined in life in the heroic work of our Lord, joined in receiving the crown of martyrdom on the same day, and joined in honor in heaven, it is tradition that the two Founders of the Roman Church can never be divided.
(Source: Interview with Fr. Martin Skierka)