Orientalium Ecclesiarum

The faithful are bound to take part on Sundays and feast days in the Divine Liturgy or, according to the regulations or custom of their own rite, in the celebration of the Divine Office. That the faithful may be able more easily to fulfill their obligation, it is laid down that the period of time within which the precept should be observed extends from the Vespers of the vigil to the end of the Sunday or the feast day. The faithful are earnestly exhorted to receive Holy Communion on these days, and indeed more frequently-yes, even daily.