Since this is observed today and tomorrow, I figured I'd post a little info for those that might be interested.
This comes from Judaism 101:
And the following comes from ou.org, who got it from...
This comes from Judaism 101:
In Hebrew, Rosh Chodesh means, literally, "head of the month" or "first of the month." Rosh Chodesh is the first day of any new month. If a month is 30 days long, then the 30th day is treated as part of the Rosh Chodesh for the next month, and the Rosh Chodesh for next month extends for two days (the 30th of the earlier month and the 1st of the later month).
In ancient times, Rosh Chodesh was a significant festival day. At that time, the new months were determined by observation. Each month began when the first sliver of moon became visible after the dark of the moon. Observers would watch the sky at night for any sign of the moon. If they saw the moon, they would report their sightings to the Sanhedrin, which would interrogate them to make sure that they were not mistaken. Where in the sky did the moon appear? Which direction was it pointing? If two independent, reliable eyewitnesses confirmed that the new moon had appeared and described it consistently, the Sanhedrin would declare the new month and send out messengers to tell people when the month began.
The day after the moon appeared was a festival, announced with the sounding of the shofar, commemorated with solemn convocations, family festivities and special sacrifices. The importance of this holiday in ancient times should not be underestimated. The entire calendar was dependent upon these declarations; without the declarations, there would be no way of knowing when holidays were supposed to occur.
In later days, however, the calendar was fixed by mathematical computation. After the destruction of the Temple, sacrifices were no longer available. Accordingly, the significance of this festival has substantially diminished. There are some slight changes to the liturgy for Rosh Chodesh, including the addition of part of Hallel after the Shemoneh Esrei, and some additional Torah readings, but that is about the only observance of Rosh Chodesh today.
It remains a custom in some communities for women to refrain from work on Rosh Chodesh, as a reward for their refusal to participate in the incident of the Golden Calf.
Shabbat Mevarekhim
The shabbat before Rosh Chodesh is known as Shabbat Mevarekhim, which means "the sabbath of blessing." After the Torah reading in the shabbat service, the prayer leader holds the Torah scroll, recites a blessing hoping for a good month, then announces the day of the upcoming week when the new month will begin and the name of the new month.
And the following comes from ou.org, who got it from...
(Material below is adapted from Mesorah Publications' Stone Edition of the Chumash)
"This is the very first commandment given to the nation as a whole, an indication that the concept of Rosh Chodesh, or the New Moon, is very meaningful. Moreover, a thousand years later in Eretz Yisrael, during the period of Syrian-Greek persecution that culminated in the miracle of Chanukah, Rosh Chodesh was one of only three commandments whose observance the oppressors prohibited. The other two forbidden commandments were the Sabbath and circumcision; that Rosh Chodesh was on a plane with those central observances is sufficient indication of its great significance."
"This can be understood on two levels. As will be noted below, only the court can proclaim Rosh Chodesh based on the testimony of witnesses who observed the re-appearance of the moon, and upon this proclamation, the Jewish calendar is based. Unless the new months can be proclaimed, there is no calendar, and without a calendar, there can be no festivals. Thus, if the Syrian-Greeks had succeeded in eradicating the observance of Rosh Chodesh, they would have succeeded in eliminating large numbers of other mitzvot, as well."
"On a deeper level, Rosh Chodesh symbolizes renewal, the ability of the Jewish People to rise up from oblivion and restore itself to its past greatness. Just as the moon disappears at the end of each month, but returns and grows to fullness, so Israel may suffer exile and decline, but it always renews itself - until the coming of the Messiah, when the promise of the Exodus and the Revelation at Sinai will be fulfilled, never to be dimmed again."
"This essential characteristic of Jewish history was first exhibited in Egypt, when, in the simile of the Sages, the nation had fallen to the forty-ninth level of impurity - one level above spiritual extermination - only to renew itself so breathtakingly that after seven weeks it was able to stand at Mount Sinai and experience prophecy."
"This concept of Jewish renewal was what the Syrian-Greeks attempted to eradicate by ending the observance of Rosh Chodesh. Instead, the Jewish People rose up in defense of the Torah and the Temple, and their triumph is commemorated through Chanukah, the festival of renewal."