The Sfas Emes explains that Shabbos represents the moment when Hashem Himself opens the gate from Above, drawing the world into a realm that surpasses nature entirely. This gift is entirely His initiative. This is a kedushah that predates our arrival on the scene entirely - it was woven into Creation from the very sixth day, when Hashem rested and blessed and sanctified the seventh. We receive it; we do not make it.
But Rosh Chodesh is different. "וּבְיוֹם הַחֹדֶשׁ יִפָּתֵחַ" - on the day of the new month it shall be opened - by us. The Sfas Emes likens it to a lock with two keys. One is held by the Ribbono Shel Olam; the other has been entrusted to Klal Yisrael. The two steps to "opening the lock" are hinted at in the pesukim "פִּתְחוּ לִי שַׁעֲרֵי צֶדֶק" - open for me the gates of righteousness (תהלים קיח יט), and "פִּתְחִי לִי אֲחֹתִי" - open for me, my sister (שיר השירים ה ב). Two verses, two kinds of opening - one from Above, one from below. And this is the meaning of "הַחֹדֶשׁ הַזֶּה לָכֶם" - this month is for you. The power to consecrate time, to turn the renewal of the moon into an opening of the heavens, has been placed in our hands.
The Sfas Emes continues that each of the twelve Shevatim holds the key to its corresponding month. And when their month arrives, that tribe's unique spiritual capacity unlocks the gate particular to it. (שפת אמת פר' החודש)
