In a recent thread some referred to a liturgical canon which prohibited more than one Euchristic Liturgy (EL) per day. As far as I am aware, current OO practice only prohibits more than one EL per-altar/per-priest per day, which hence allows for more than one EL per day at any church with more than one altar and more than one priest.
What are the sources of the canons in question (pre-schism? post-schism?) and the justifications underlying them (patristic justifications would be preferable)?