8/13/2015, at even began the New Moon observance. Each new moon marks the beginning of a new month. The term month is derived from the term month.
The Law (1st 5 Books of the scripture) speaks of the mandate to observe the New Moon:
Numbers 10:10 Also in the day of your gladness, and in your solemn days, and in the beginnings of your months, ye 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 God: I am YHWH your God.
The scriptures support the mandate of the New Moon observance:
Psalms 81:3 Blow up the trumpet in the new moon, in the time appointed, on our solemn feast day.
Isaiah 66:23 And it shall come to pass, that from one new moon to another, and from one sabbath to another, shall all flesh come to worship before me, saith YHWH .
1 Samuel 20:18 Then Jonathan said to David (King to be), To morrow is the new moon: and thou shalt be missed, because thy seat will be empty.
The dates of the monthly new moons are the keys to observing the seven annual mandated Holy Days listed in Leviticus chapter 23.