What is kiddush hachodesh (blessing of the new month)? Why are we mekadesh the chodesh? What does it involve and why do we announce the molad? Do need to know the molad at the time of the prayer?
Chodesh literally means new moon. Each month starts with the rebirth of the moon, waxing till the middle of the month, then waning for the next fifteen days, till it disappears and the cycle repeats itself with the next ‘new moon’. The molad is the time of the birth of the New Moon.
Astrologically, the moon is ‘dead’ at the time of its rebirth; it cannot reflect or direct influences, either of itself or of the other planets and stars. Specifically at this time, when it is סמוי מן העין – hidden, at the moment of its rebirth, we can ask G-d to bless the new moon , that all astrological influences for the coming month be good ones, blessed by G-d’s favour. That is the reason behind announcing the molad: we need to know the molad in order to bless that moment of the ‘rebirth’ of the new moon.
[Like all Mitsvos, the meaning behind Kiddush hachodesh is profound and multi-faceted but the above significance is relevant on all levels. On a higher level, for example, there is a renewing, as it were, of the particular influx from the Shechinoh, peculiar to that month: this is part of the deeper implication of Kiddush HaChodesh.]
The subject of astrological influence vis-à-vis the Jewish people is largely misunderstood. Besides the fact that most Rabbis of the Mishnah, Talmud and later commentaries are of the opinion that the Jewish people are not [both individually and, according to the Ibn Ezra and Bnei Yissosschor, collectively] beyond astrological influence , even the oft-quoted saying of R’ Yochanon (Shabbos 156a) that אין מזל לישראל – there is no such thing as astrological influence [mazal = influence of a constellation] for Yiden – in other words, that Jews are apparently above astrological influence, is explained by Rabbi Chaim Vital  and others to mean that though there is undoubtedly astrological influences on each person  [he goes as far as to write that every happening during a person’s life can be [fore]seen in the stars], each spiritual world has its mazal, so since the Jewish soul is rooted higher than the stars and their influence, the higher worlds’ mazal, the influence of the world of souls, etc, can override any astrological influence. But this overriding does not mean it ignores the astrological process; it is a higher influence that is then channelled through the mazolos of the constellations. (On a practical level, this means that a person is still likely to express him or herself according to the arrangement of the mazolos at the time of birth and the subsequent planetary aspects.)
Coming back to Kiddush HaChodesh. ‘Sanctifying’ the New Moon means asking G-d to ‘bless’ the new moon; through our tefillos, all astrological influences are now subjugated to the higher holy worlds – of the holy angels, the soul and higher still. On a certain level, this appears to be at play too when we are mekadesh the levonnoh , and that is why according to the Gemoro [inferred from the discussion there], Semag, Rambam, [Rabbeinu Yoinoh,] and Arizal , it is important to be mekadesh the levonoh as early as possible – striking the iron while it’s still hot. Astrologically, the moon is responsible for directing and expressing the influences of the other planets to the individual. By being mekadesh the chodesh and levonnoh, we realise all influences to be directed by HaShem and thus repel any untoward outcome.
 This is why we say in Hallel, ‘זה היום עשה ד', נגילה ונשמחה בו’: the time of the New Moon is Kabbalistically a time ripe to receive heavenly blessing as all influence is סמוי מן העין [hidden] ואין הברכה שורה אלא בדבר הסמוי מן העין (ב"מ מו:) – blessing can only come when it is hidden.
 The Rambam’s opinion appears to be the exception though even he appears to acquiesce that there is astrological influence; he merely says that to try to interpret the stars or ‘foresee’ events is futile.
 Shaarei Hakedushoh
 As can be seen quite clearly from the proof that Rav brings – see Shabbos ibid.
 ספר התכונה
 Kabbalistically, kiddush halevonoh has much deeper significance, effects and meanings, beyond the scope of this short article. On the kabbalistic level, the moon corresponds to the Shechinoh, the Divine Presence, which is responsible for the blessings that accompany the Jewish People.
 The exact opinion of the Ari HaKodosh is a subject of dispute. From Shaar HaKavonos and Pri Eits Chaim it is apparent that the Arizal adopted the opinion of the Rambam, contrary to the opinion of earlier kabbalists and the prevalent minhag. However, the Rashash Hakodosh waited seven days from the beginning of the month till being mekadesh the levonnoh; as he never veered from the Ari in his customs and opinions, it appears he knew of another source in the Ari, unknown to us. A fuller analysis of this dispute is given in the sefer עמודי הוראה by Rabbi Yaakov Hillel שליט"א.