Bnei Yisrael rise above nature

בפסוק החדש הזה לכם כו'.

On the pasuk, "This month shall be for you…"

The Sefas Emes opens with the first mitzvah given to Bnei Yisrael — sanctifying the new month — beginning the month of Nissan.

דאיתא באחד בניסן ר"ה למלכים כי תשרי ר"ה לשנים הוא התחדשות השנה עפ"י דרך הטבע ובניסן ר"ה למלכים למלכי ישראל כדאיתא בגמ' והיינו בני ישראל שנקראו בני מלכים שהם בני חורין משיעבוד הסט"א שאינם תחת הטבע ולכן אחר שהתחילה הגאולה במצרים אז נאמר להם החודש הזה לכם כו' [נשכח ממני הסיום]:

For it is taught that on the first of Nissan is Rosh Hashanah for kings — since Tishrei is Rosh Hashanah for years, which is the renewal of the year according to the way of nature, whereas Nissan is Rosh Hashanah for kings, for the kings of Yisrael, as is brought in the Gemara. And this refers to Bnei Yisrael, who are called "children of kings," for they are free men from the bondage of the Sitra Achra (the "other side") and are not under nature. Therefore, after the redemption began in Mitzrayim, it was said to them, "This month shall be for you…" [the conclusion has slipped from my memory].

The two Rosh Hashanahs reflect two orders. Tishrei governs the natural year — the cycle of nature into which all the world is bound. Nissan governs the "kings of Yisrael," and Bnei Yisrael are "children of kings," bnei chorin (free) from the grip of the Sitra Achra and not subject to the rule of nature. Once the geulah from Mitzrayim began, lifting them above nature, Hashem could give them the mitzvah of "hachodesh hazeh lachem" — the power to sanctify time itself, marking that they now stand above the natural order. (The Rebbe notes he no longer recalls how the thought concluded.)

Summary: Nissan is Rosh Hashanah for the kings of Yisrael because Bnei Yisrael are "children of kings," free from the bondage of the Sitra Achra and not bound by nature. The geulah from Mitzrayim raised them above the natural order, so that with "hachodesh hazeh lachem" they were given mastery over time itself.