Numbers 4:7 tells us that the shewbread was to be continual and not only for the Sabbaths.
7 And upon the table of shewbread they shall spread a cloth of blue, and put thereon the dishes, and the spoons, and the bowls, and covers to cover withal: and the continual bread shall be thereon:
4 Behold, I build an house to the name of the Lord my God, to dedicate it to him, and to burn before him sweet incense, and for the continual shewbread, and for the burnt offerings morning and evening, on the sabbaths, and on the new moons, and on the solemn feasts of the Lord our God. This is an ordinance for ever to Israel.
Claim” David ate shewbread upon the Sabbath”
1 Samuel 21
6 So the priest gave him hallowed bread: for there was no bread there but the shewbread, that was taken from before the Lord, to put hot bread in the day when it was taken away.
The day you claim was a Sabbath was not. The Bible does not say it was a Holy convocation, only that David ate of the continual bread which was for the priests only.