The Feast of the Twelve Apostles (or Synaxis of the Twelve Apostles) is a Christian holiday celebrated on June 30 (on both the New and Old Calendars) to honor all twelve of Jesus Christ's primary disciples together.
It follows the Feast of Saints Peter and Paul on June 29, commemorating their collective mission and the founding of the Church.
You can visit the Church of the Twelve Holy Apostles in Rome.