By: Rabbi Ari Enkin
There is an ancient custom to bless the upcoming Jewish month on the Shabbat which precedes Rosh Chodesh. These Shabbatot when the new month is blessed are referred to as “Shabbat Mevarchim”. A central feature of this prayer is the public announcement of the exact time the new moon, known as the “molad”, is set to appear. The molad is the exact moment of the moon’s renewal. Recall that at the concluding moments of every lunar cycle the moon is positioned directly between the earth and the sun and is completely invisible. A few hours following this occurrence the moon is once again visible, albeit only the most minute portion of it. At such time the "molad" the "birth" is recognized as having taken place and a new Jewish month has begun.
The prayer for the new month is conducted in a somewhat distinguished manner. It is led by the Chazzan while holding the Torah. The prayer announces the name of the upcoming month along with the exact moment the new moon is set to appear. It also serves to alerts the congregation as to which day(s) of the coming week will be observed as Rosh Chodesh. Finally, it asks that God renew the upcoming month with all forms of blessing. It is often carried out in an elaborate and impressive manner.
It is actually quite important that the exact time of the molad be announced. Doing so allows people to determine when the latest opportunity is for them to recite the Kiddush Levana. It also represents our efforts to familiarize ourselves with the workings of the calendar. Indeed, we are strongly encouraged to somehow participate in the calculating of celestial matters. Yes, learning the basics of astrology is a mitzva! 
Announcing the molad also recalls the ancient practice of “Kiddush Hachodesh”. In Temple times the new month was declared by the Beit Din based on manual calculations along with the testimony of witnesses who would have claimed to have seen the moon’s renewal. It is rather essential to be standing when reciting this prayer as the ancient Kiddush Hachodesh was only acceptable if performed while standing. Although it is technically to recite the "Mevarchim" prayer on any day of the week preceding Rosh Chodesh, universal custom is to recite it on Shabbat when more people are to be found in the synagogue. Indeed, it is always commendable to make an effort to perform mitzvot in the presence of as many people as possible.
There is one month, however, that is excluded from the cycle of Shabbat Mevarchim prayers and that is the month of Tishrei. Among the many explanations as to why this so, it is said that the month of Tishrei with all its holidays and their accompanying mitzvot is inherently blessed. It is as if God Himself is the one who blesses the month of Tishrei. The other months of the year which don't contain nearly as much mitzvot as Tishrei are therefore dependant upon the Jewish people to bless them.
While in our day Tishrei is the only month for which Shabbat Mevarchim prayers are not recited, there was once a custom to omit them for the month of Av, as well. While no longer practiced, the idea for omitting the prayer on behalf of the month of Av was related to that month’s eternal association with bad fortune and tragedy. It is actually the opposite reasoning that has prevailed however – being that the month is so associated with tragedy, how much more so does it require our prayers!
 Mishna Berura 417:1
 Kaf Hachaim 417:1
 Shabbat 75a
 Rashi, Maharsha, Rambam ad loc.
 Magen Avraham 417:1, Kaf Hachaim 417:7, Igrot Moshe 1:142
 Likutei Maharich;Rosh Chodesh
 Rosh Hashanah 32b, Pesachim 64b, Menachot 64a
 Likutei Sichot;Nitzavim-Vayelech 5744
 Magen Avraham 417:1
 Machatzit Hashekel O.C. 417
 Ateret Zekeinim 549