Rosh Hashanah - the first day of Tishrei - is referred to in Scripture as "the day of concealment," as the verse states (Tehillim 81:4): Sound the shofar to mark the new month, the time of concealment of our Festival day.

All that transpires on Rosh Hashanah has an element of concealment. The Talmud (Beitzah 16b) states: Sound the shofar to mark the new month, the time of concealment - which Festival falls when the new moon is still concealed? Rosh Hashanah. All other Festivals fall either on or nearer the time of the full moon, whereas Rosh Hashanah falls when the moon is concealed.

Israel is compared to the moon and she is radiant on the Festivals . . . on Rosh Hashanah, however, she diminishes herself and conceals her greatness in trepidation of the Day of Judgment. In the same manner, G‑d conceals her sins and accords her forgiveness (Pesikta Rabbati 40).

The very character of the first of Tishrei as a Day of Judgment is similarly concealed and is not expressly mentioned in the Torah. The reason is that man should concern himself with his sins all year and not delay his repentance until Rosh Hashanah.

This element of concealment also finds expression in our custom not to recite the blessing over the new month on the Shabbat before the first of Tishrei. The reason is that we thereby conceal the approaching Day of Judgment from Satan, so that he might not come and prosecute Israel for her sins.