Charles Ellicott Commentary 1 Samuel 20:6

"If thy father miss me at all, then say, David earnestly asked leave of me that he might run to Beth-lehem his city; for it is the yearly sacrifice there for all the family." — 1 Samuel 20:6 (ASV)

A yearly sacrifice. —The Mosaic Law (Deuteronomy 12:5 and following verses) strictly required these great sacrificial feasts to be kept at the Tabernacle, “unto the place which the Lord your God shall choose out of all your tribes;” but ever since the destruction of the Tabernacle of Shiloh there had been no central sanctuary, and these solemn feasts had been held, most probably, in tribal centres. “In the then-disorganized condition of public worship to which David first gave regular form, family usages of this sort, after the manner of other nations, had established themselves, which were contrary to the (Mosaic) prescriptions concerning the unity of Divine worship.”—O. von Gerlach, in Lange.

It is highly probable that the festival in question was at this time being held at Bethlehem. It is, however, clear that David did not intend to be present at it, and therefore the excuse was a feigned one. The morality of this request of David is by no means sanctioned by the compiler of the history; he simply relates the story.