http://revach.net/article.php?id=4751

Section: Moadim   Category: Around The Year
Mogen Avrohom - Shabbos Before Rosh HaShanah Is Not Shabbos Mivarchim
Every Shabbos before Rosh Chodesh we are Mivarech the Chodesh.  We add an extra Tefila announcing the new month and asking Hashem for it to be a good one.  The Mogen Avrohom brings (417:1) from the Yerei'im that this practice does not constitute Kiddush HaChodesh, where Bais Din has the power to declare a new month, but rather it is just an announcement to let everyone know that Rosh Chodesh is coming.

It is for this reason says the Mogen Avrohom that we are not Mivarech Tishrei on the Shabbos before Rosh HaShanah.  The pasuk says "Tiku BaChodesh Shofar Bakeseh L'Yom Chageinu".  Rosh HaShanah is the Chag that is Keseh, hidden.  It comes under the cover of darkness since it takes place on the first day of the month when the moon shines the least bright and for the shortest amount of time.  In continuing this idea we don't make any announcements about its imminent arrival on the Shabbos before.