As we've discussed before, Parshas haChodesh has actually very little to do with Rosh Chodesh. After the first two pesukim, the rest of the parsha deals with the laws of the korban Pesach, with the first seder ever conducted. Why did the Torah even need to introduce the laws of Rosh Chodesh here instead of at mattan Torah?
There is a basic difference between Shabbos and Rosh Chodesh. Shabbos is the culmination of the work week. Chazal tell us that only someone who prepares on erev Shabbos, who uses the time before Shabbos properly, will eat on Shabbos. If you see a nice roast in the market on Monday, you should put it aside for Shabbos in advance. You are even allowed to do work to prepare for Shabbos before davening on Friday because there is a mitzvah of kavod Shabbos to prepare for Shabbos. Not so Rosh Chodesh, which is celebrated in anticipation of the month ahead, not as a culmination of what already took place.
There is a minhag quoted in Shulchan Aruch for women not to do work on Rosh Chodesh. According to the Da'as Zekeinim in last week's parsha women earned this reward because they gave their jewelry to the Mishkan, which was erected on Rosh Chodesh. The mirrors which the women used in Mitzrayim to show their husbands their beauty and encourage them to have children and not give up hope were incorporated into the making of the kiyor. This anticipation of the future geulah symbolized by those mirrors is the hallmark of Rosh Chodesh, the holiday of anticipation of what is yet to come. R' Tzadok haKohen explains that the renewal of the moon on Rosh Chodesh symbolizes the renewal of malchus beis David. Women already celebrate the day as a Yom Tov because to them the future geulah is real, their anticipation makes it seems as if geulah is already here, even if b'poel it has not happened yet.
Bnei Yisrael were bereft of mitzvos and merits when the time came for their geulah. Therefore, explains the Shem m'Shmuel, Hashem gave them the mitzvah of Rosh Chodesh, which illustrates that it's not just what you have done in the past that counts in your favor, but it's what's coming in the future as well.