The end of Parshat Mishpatim lists several laws relating to the Chagim, one of which is “Lo Yalin Chelev Chagi Ad Boker,” “You shall not leave the fat of my ‘Chag’ until morning” (23:18). According to the Gemara in Pesachim (59b), the Pasuk is instructing that when the Korban Chagigah for Pesach is brought, one cannot wait until morning to place the Korban’s Chelev (fats) on the altar. However, the rationale for this commandment is unclear. The Netziv therefore suggests that the commandment is intended to counteract the natural reasoning of the person offering the Korban. We know that the daytime is considered a more important time for acceptance of prayer, as the Pasuk in Tehillim (88:2) says, “Yom Tzaakati Balayla Negdecha Tavo Lifanecha Tefilati,” suggesting that if a person has prayed all night, his prayer is then accepted the next day. Since the Korban is brought as a form of Tefillah, the person offering the Korban would reason that the burning of the Chelev on the altar, the part of the Korban most connected to Tefillah, should occur during the day. Thus, the Pasuk instructs that one cannot wait until the daytime to place the Chelev on the altar; he must place the Chelev on the altar to burn during the night.
Rav Shimshon Raphael Hirsch suggests a different rationale for the Mitzvah. He writes that the Chelev represents material aspirations and desires. This symbolism stems from the fact that an animal stores Chelev for its future physical needs. Moreover, Rav Hirsch points out that the Kelayot (kidneys) are also placed on the altar with the Chelev. These, too, signify physical desires, since the word “Kelayot” is related to “Kalah,” “to long for,” and the kidneys are often associated in the Torah with the heart, which is obviously related to desire. The Torah therefore commands that the Chelev of a Korban must always be brought on the same day as the blood of the Korban, which symbolizes spiritual needs (as is written in the Pasuk in Vayikra 17:11). Thus, the part of the animal that symbolizes physical needs will be brought on the same day as the part that symbolizes the spiritual. By bringing the Chelev on the same day as the blood, we show that we do not believe that our material desires are unrelated to worshipping God; on the contrary, we believe that we can harness them to serve the same purpose as our spiritual desires. Furthermore, the commandment of “Lo Yalin Chelev Chagi Ad Boker” is specifically mentioned regarding the Korban Chagigah, a Korban which unifies all of Bnei Yisrael as one nation. Thus, we must put the Chelev on the altar on the same day as the blood of the Chagigah to show that we must use our material desires to serve God even at the national level. A nation may not seek its material desires for an immoral purpose, even though some might justify it because it benefits the entire community. Rather, even at the communal level, both material and spiritual desires must help Bnei Yisrael to come close to God.