Shalom, Why arent New Moons or Rosh Chodesh considered a time for cessation of work in the Torah? Also in light of Amos 8:5 where HaShem rebukes those doing business on Rosh Chodesh, why do we, or more accurately why are we allowed to work on Rosh Chodesh?

The Torah specifies (see for instance Leviticus ch 21) all the days on which melachah is forbidden, but does not specify Rosh Chodesh, not once. Rav Soleveitchik explained that the holiness of Rosh Chodesh pertains to the holiness of everyday affairs, so cessation of work would contradict the nature of the day. As to Amos 8;5, check out the classical commentaries. Rashi, for instance says the verse is referring to "ibbur Shane" adding an extra month to the year.