The reason why a Vigil is appointed as the Sabbath day dawns, and why a dispensation from fasting is enjoyed on the Sabbath all through the East.
And throughout the whole of the East it has been settled, ever since the time of the preaching of the Apostles, when the Christian faith and religion was founded, that these Vigils should be celebrated as the Sabbath dawns, 749 for this reason,—because, when our Lord and Saviour had been crucified on the sixth day of the week, the disciples, overwhelmed by the freshness of His sufferings, remained watching throughout the whole night, giving no rest or sleep to their eyes. Wherefore, since that time, a service of Vigils has been appointed for this night, and is still observed in the same way up to the present day all through the East. And so, after the exertion of the Vigil, a dispensation from fasting, appointed in like manner for the Sabbath by apostolic men, 750 is not without reason enjoined in all the churches of the East, in accordance with that saying of Ecclesiastes, which, although it has another and a mystical sense, is not misapplied to this, by which we are charged to give to both days—that is, to the seventh and eighth equally—the same share of the service, as it says: “Give a portion to these seven and also to these eight.” 751 For this dispensation from fasting must not be understood as a participation in the Jewish festival by those above all who are shown to be free from all Jewish superstition, but as contributing to that rest of the wearied body of which we have spoken; which, as it fasts continually for five days in the week all through the year, would easily be worn out and fail, unless it were revived by an interval of at least two days.
The observance of a vigil for the whole or greater part of the night was a regular part of the preparation for the greater festivals, and as such was usual in the East before the Sabbath (Saturday) and Lords Day, as well as Pentecost and Easter. See Socrates, H. E. VI. viii., where there is an allusion to this.217:750
Saturday, as well as Sunday, was long regarded as a festival in the East, and, indeed, originally in most churches of the West as well. See the Apost. Const. II. lix. 1; VIII. xxxiii. 1. Apost. Canons lxvi.; Council of Laodicæa, Canons xvi., xlix., li.217:751
Eccl. xi. 2.