The reason for not observing many customs from Shabbos Mevorchim Chodesh Tishrei through Rosh Hashana is enlightening. They are: blessing the new month, mentioning on Rosh Hashanah that it is Rosh Chodesh, starting the beginning of the Torah on Rosh Hashanah and blowing the Shofar erev Rosh Hashanah. This is to weaken the resolve of the Satan, since he sees that we are so confident that we are not even observing these things. In a deeper sense, the pain that results for us by not doing these things (because we don’t want the Satan to prosecute us) brings us to a very deep level of Tshuva.

לקו״ש כד