The tenth chapter of Numbers is the first mention in Scripture about New Moon.

vs. 10 Also in the day of your gladness, and in your solemn days, and in the beginnings of your months, you shall blow with the trumpets over your burnt offerings, and over the sacrifices of your peace offerings; that they may be to you for a memorial before your Elohim: I am YHVH your Elohim.

The beginnings of your months are the New Moons. The context of this chapter is that of trumpets and their purpose.

vs. 1 And YHVH spoke unto Moses, saying,

vs. 2 Make you two trumpets of silver; of a whole piece shall you make them: that you may use them for the calling of the assembly, and for the journeying of the camps.

vs. 3 And when they shall blow with them, all the assembly shall assemble themselves to you at the door of the tabernacle of the congregation.

vs. 4 And if they blow with one, then the princes, heads of the thousands of Israel, shall gather themselves unto thee.

vs. 5 When you blow an alarm, then the camps that lie on the east parts shall go forward.

vs. 6 When you blow an alarm the second time, then the camps that lie on the south side shall take their journey: they shall blow an alarm for their journeys.

vs. 7 But when the congregation is to be gathered together, you shall blow, but you shall not sound an alarm.

vs. 8 And the sons of Aaron, the priests, shall blow with the trumpets; and they shall be to you for an ordinance forever throughout your generations.

vs. 9 And if you go to war in your land against the enemy that oppresses you, then you blow an alarm with the trumpets; and you shall be remembered before YHVH your Elohim, and you shall be saved from your enemies.

YHVH instructed Moses to use trumpets for two purposes as recorded in vs. 2: 1) the calling of the assembly (as in Joel 2:15-17), and 2) the journeying of the camps (as in Jeremiah 4:5).

YHVH uses repetition to teach these purposes in the same order as given in verse 2, alternating back and forth: 1) the calling of the assembly (vs. 3 and again in vss. 7-8), and 2) the journeying of the camps as an alarm (vss. 4-6 time to move on and vs. 9 time for war).

Verse 10 is a notation to also blow the trumpet over the offerings of your special days that include the New Moon. This is a time to gather together before YHVH!

Numbers 29:1 set forth the Feast of Shoutings (Trumpets) as a day of "holy convocation" or gathering together per Numbers 10:7 in which ". . . you shall do no servile work: it is a day of blowing trumpets to you." The implication here is that if the trumpet was blown for services, then the day was at least treated like a Sabbath in which no servile work was performed.

The Scriptural record for the observance of New Moon is as a day to gather together for worship in joyful celebration, and to cease from work and business. Although this day is never called out in Scripture to be a Sabbath, it is at least treated as a Sabbath by application.