Synaxis of the Twelve Apostles:   a synaxis is a  commemoration of a group of saints associated with a particular feastday.  It takes place on the day after the main feast, so, after Peter and Paul are honored, the Twelve are then also honored.  We say ‘thank you’ after someone does something special for us…this is a Liturgical thank you.

http://oca.org/saints/lives/2015/06/30/101711-synaxis-of-the-holy-glorious-and-all-praised-twelve-apostles

http://images.oca.org/icons/sm/june/0630.12Apostles2.jpg