On pages 135-136 of his chapter on "Halakha" in The Literature of the Sages, Part I, Shmuel Safrai talks about halakhah in the Hebrew Bible. Here are a couple of examples:
1. In Exodus 30:11-16, the Israelites are commanded to contribute half of a shekel to ward off a plague that could result from a census. This was a command for a "one time payment" (Safrai's words). But, somehow, it got "turned into a regular contribution." II Chronicles 24:6ff. mentions Josiah collecting "the tax levied by Moses," and Nehemiah 10:33f. refers to "the third part of the shekel for the service of the house of our God" (Safrai's quotations of the biblical texts). Second Temple sources and the Mishnah refer to a commandment of the half-shekel, and, "after a long struggle between Pharisees and Sadducees", "the donation of the half-shekel became a matter of principle" "during the Hasmonean period."
2. According to Safrai, "The Tora...does not mention any specific prohibition of labour on the New Moon day, other than its special sacrifice (Num 28:11-15)." But, in Amos 8:5, "the New Moon and the Sabbath are mentioned together as days when one should refrain from commerce", a sentiment which also appears in a baraita in Babylonian Talmud Meg. 22b. In Isaiah 1:13, new moons are times when Israelites gathered in the Temple, and this sort of thing will also occur after Israel's restoration (Isaiah 66:23). In II Kings 4:23, new moons are times when people can see a prophet.
I found this interesting on account of my own Armstrongite background, in which I rested on the Sabbath and holy days. What's ironic is that I didn't "rest" as much as orthodox Jews, and yet there were areas in which I rested more than the halakhah required! For example, Jews are allowed to read on the Sabbath, but they cannot write, and so there are Jewish academics who feel that they are not transgressing the Sabbath when they read scholarly journals.
But, anyway, I wondered if I had to rest on new moons as well. I knew one person who wanted to take the new moons off from work. But the Armstrongite ministers whom I heard said that there was no Scriptural command to rest on the new moons. Actually, that appears to be only half right: there is no command in the Torah to treat the New Moon as a sort of Sabbath, but Amos 8:5 seems to presume precisely that. And the new moons were times of worship---not necessarily mandatory worship, but Israelites did worship on those days.
That may have actually put me in a dilemma in the old days. Would I have to take off work or school on the new moons? I already had to take off enough work or school by observing the holy days! Nowadays, however, my spirituality is not based on keeping days. I myself refrain from work on the Sabbath, because I need rest and worship. I also try to honor the holy days. But I don't think that God imposes these things on me as mandates---as if I won't enter the kingdom of God if I don't observe them.