Canon 15
Concerning the celebration of Masses, particularly on the occasion of special festivities, it should be observed that the celebration of Masses starts in the name of God at the third hour [i.e. terce, ca 9 a.m. depending on the time of year]. In this way, it would be easier for priests to gather at the evening services, that is at the time of vespers, after having carried out their duty in the meantime. For it is both unbecoming and unfitting for a priest not to attend the evening services at the church, especially on such particular days.
(trans. J. Szafranowski)