Adam Clarke on 1 Samuel 20:5

To-morrow is the new moon - The months of the Hebrews were lunar months, and they reckoned from new moon to new moon. And as their other feasts, particularly the passover, were reckoned according to this, they were very scrupulous in observing the first appearance of each new moon. On these new moons they offered sacrifices, and had a feast; as we learn from Num 10:10; Num 28:11. And we may suppose that the families, on such occasions, sacrificed and feasted together. To this David seems to refer; but the gathering together all the families of a whole tribe seems to have taken place only once in the year. There is a yearly sacrifice there for all the family, 1Sam 20:6.