1 Sam 20 - They celebrated the new moon for 2 days, it can only be estimated beforehand. 2023

1 Sam 20:34
Then Jonathan arose from the table in fierce anger, and did not eat food on the second day of the new moon, for he was grieved over David because his father had dishonored him.

Jonathan left the table and was grieved because of his father. Apparently they celebrated the new moon for two days, but there is nothing in Torah about that. It could be that they did so because the precise day of the New Moon could only be estimated beforehand and this assured them that they would hit the right day.

1 Chron 23:30-31 And they are to stand every morning to thank and to praise the LORD, and likewise at evening, 31and to offer all burnt offerings to the LORD, on the sabbaths, the new moons and the fixed festivals in the number set by the ordinance concerning them, continually before the LORD.