John Cassian, The Institutes III.12: Of the days on which, when supper is provided for the brethren, a Psalm is not said as they assemble for the meals as is usual at dinner

Of the days on which, when supper is provided for the brethren, a Psalm is not said as they assemble for the meals as is usual at dinner.

Lastly, also, on those days,—i.e., on Saturday and Sunday,—and on holy days, on which it is usual for both dinner and supper to be provided for the brethren, a Psalm is not said in the evening, either when they come to supper or when they rise from it, as is usual at their ordinary dinner and the canonical refreshment on fast days, which the customary Psalms usually precede and follow. But they simply make a plain prayer and come to supper, and again, when they rise from it, conclude with prayer alone; because this repast is something special among the monks: nor are they all obliged to come to it, but it is only for strangers who have come to see the brethren, and those whom bodily weakness or their own inclination invites to it.