Question
Chodesh Tov, why didn't we bless the month of Tishrei?
Answer
ב"ה
Shalom
Right now, we are in the month Elul, and Rosh Hashana which is the first of Tishrei, will also be Rosh Chodesh.
The Talmud (Rosh Hashana 8a-b) asks: "Which holiday falls out on a day when the "Chodesh" moon is concealed?" And the Talmud answers Rosh Hashana. This is because Rosh Hashana is the only holiday which falls at the beginning of the month, when the moon is not seen.
The motif of being concealed repeats itself on Rosh Hashana in many aspects. For example, the whole idea that Rosh Hashana is a day of judgement is not mentioned in the Torah.
Another idea of concealment is the idea that the Jewish people are compared to the moon which shines bright on Shabbat and other holidays. However, on Rosh Hashana, which a day of judgement, we choose to conceal ourselves as it were, because of the fear of Hashem's judgement. (Sefer Hatoda'ah= The Book of our Heritage Rav Eliyahu Ki-tov zt"l )
Similarly, we pray that g-d will conceal our sins.
Another reason offered, is that of the Levush ( 581), that we don't bless Rosh Chodesh Tisrei is to conceal the fact form the Satan that Rosh Hashana is coming, so that the satan should not find fault with us.
All the best
