Purity drawn down through Torah

חז"ל קבעו לקרות פרשת פרה בסוף השנה להיות כ"א בא לטהר.

Chazal established the reading of Parshas Parah at the end of the year, so that everyone may come to purify himself.

The Sefas Emes notes that the Sages fixed the public reading of Parshas Parah at this season — the close of the year before Pesach — precisely as a time when each person is meant to seek taharah (purity).

כי מסוגלים הימים הללו שנטהרו כל בני ישראל קודם הפסח ממילא מתעורר השפעת טהרה בלבות בנ"י מן השמים כמאמר הבא לטהר מסייעין אותו.

For these days are especially suited to it, since all of Bnei Yisrael purified themselves before Pesach; consequently an influx of taharah is awakened in the hearts of Bnei Yisrael from Heaven, as the teaching goes: "One who comes to purify himself is helped."

Because the generation that left Mitzrayim purified themselves with the parah adumah before that first Pesach, these days carry a permanent spiritual potency. A flow of purity is stirred from Above into Jewish hearts each year at this time — in keeping with the principle that Heaven assists whoever takes the first step toward purity.

ועיקר הטהרה הוא בכח התורה וכמ"ש גזירה הוא מלפני לא המת מטמא כו'.

And the essence of the purification is through the power of the Torah, as it is said, "It is a decree (gezeirah) from before Me" — it is not the corpse that defiles, etc.

The true source of taharah is the power of the Torah itself. As Chazal teach regarding the parah adumah, Hashem declares it a chok, a Divine decree: it is not the dead body that intrinsically defiles, but Hashem's word — and so it is Hashem's word, the Torah, that purifies.

א"כ עיקר הטהרה יש גם עתה בקריאת פרשה זו כמאמר אמרות ה' אמרות טהורות:

If so, the essence of the purification is available even now through the reading of this parshah, as it says, "The words of Hashem are pure words" (Tehillim 12:7).

Since taharah flows from the power of the Torah rather than from the physical ashes, the same purity is accessible today through merely reading Parshas Parah. The very "words of Hashem" are pure and purifying, so engaging the parshah draws down that taharah even in the absence of the actual ashes.

Summary: Chazal set the reading of Parshas Parah before Pesach because these days, when the generation of the Exodus first purified themselves, carry an enduring potency that awakens a Heavenly flow of taharah into Jewish hearts — for whoever comes to purify himself is helped. Since true purity flows from the power of the Torah and Hashem's decree rather than from the physical ashes, that same purification is attainable today simply through reading the parshah, for "the words of Hashem are pure words."