From Rabbi Shmuel - RH DAY 1: Word document, explains about the "bakeseh"

Blast the shofar, bakesah, on the time designated as a holiday. This is a statute for Israel, as ordinance for the God of Jacob
The key word in this verse is bakesah, which means covered, i.e. when the new moon is covered and not yet visible. That is when the shofar should be blasted. This verse supports the idea that the holiday of judgment coincides with the holiday in which the moon is covered. Explains the Talmud, that holiday must be Rosh Hashanah since that is the only holiday on which the moon is covered

Book of Enoch

LXXVIII. The Sun and Moon: the Waxing and Waning of the Moon.


And on the first

13 day she is called the new moon, for on that day the light rises upon her. She becomes full moon exactly on the day when the sun sets in the west, and from the east she rises at night, and the moon shines the whole night through till the sun rises over against her and the moon is seen over against

14 the sun. On the side whence the light of the moon comes forth, there again she wanes till all the light vanishes and all the days of the month are at an end, and her circumference is empty, void of

15 light. And three months she makes of thirty days, and at her time she makes three months of twenty-nine days each, in which she accomplishes her waning in the first period of time, and in the first

16 portal for one hundred and seventy-seven days. And in the time of her going out she appears for three months (of) thirty days each, and for three months she appears (of) twenty-nine each. At night she appears like a man for twenty days each time, and by day she appears like the heaven, and there is nothing else in her save her light.

Psalms 81:3
Blow up the trumpet in the new moon, in the time appointed, on our solemn feast-day. (Holy Name Bible)

Blow in the new moon the ram's horn, at the fullness, (fully covered) on day our feast:


Psalms 104:19
He appointed the moon for seasons: the sun knoweth his going down.