Jamieson, Fausset and Brown on 1 Samuel 21:5

5. these three days--as required by law (Ex 19:15). David and his attendants seem to have been lurking in some of the adjoining caves, to elude pursuit, and to have been, consequently, reduced to great extremities of hunger.

the bread is in a manner common--that is, now that it is no longer standing on the Lord's table. It is eaten by the priests, and may also, in our circumstances, be eaten by us.

yea, though it were sanctified this day in the vessel--that is, though the hallowed bread had been but newly placed on the vessel, the ritual ordinance would have to yield to the great law of necessity and mercy (see on Mt 12:3; also see Mr 2:25; Lu 6:3).