Likkutei Etzot, Moadei Hashem, Rosh Chodesh

מוֹעֲדֵי ה' – רֹאשׁ חֹדֶשׁ
א. שֹׁרֶשׁ הַתְּשׁוּבָה הוּא רֹאשׁ חֹדֶשׁ. עַל כֵּן בְּרֹאשׁ חֹדֶשׁ מִשְׁתַּלְשֶׁלֶת הַתְּשׁוּבָה בְּכָל הַנִּבְרָאִים וּמַגִּיעַ לָהֶם הִרְהוּר תְּשׁוּבָה. וַאֲפִלּוּ רְשָׁעִים שֶׁבַּגֵּיהִנֹּם מֻכְרָחִים לְאֵיזֶה חֲרָטָה בְּרֹאשׁ חֹדֶשׁ, שֶׁשָּׁבִין וּמִתְחָרְטִין וּמוֹדִים וּמִתְבַּיְּשִׁים: (לק"א סי' י' אות ט') FESTIVALS AND SEASONS OF THE YEAR: Rosh Chodesh — The New Moon
The force which draws all the worlds to return to God has its root on Rosh Chodesh, the New Moon. On Rosh Chodesh the desire to return to God is felt over the entire creation. Even the sinners in hell cannot escape a certain feeling of contrition. They are drawn a little closer to God and feel a flicker of regret. They are forced to acknowledge the truth and feel ashamed of their wrongdoing (10:9).