The sun sets over the horizon as the day comes to an end. In the Jewish calendar, a new day is just about to begin, a fact which bears significance in so many areas of halachah.
The pasuk in Parshas Emor (Vayikra 22:7) says: “Uva hashemesh v’taher v’achar yochal min hakedoshim ki lachmo hu” - "And the sun will set, and he will become pure; afterwards, he may eat from the holy offerings, for it is his food."
Rashi explains that a kohen who became tamei must first immerse in a mikvah. However, even after his immersion, he cannot yet eat terumah. Only after ha'erev shemesh — when the sun sets — does he become fully tahor and is permitted to eat terumah.
This raises a question: If the kohen has already gone to the mikvah earlier in the day, and he will not bring any korbanos until the next day anyway, why does the Torah require him to wait for nightfall to achieve full purity? What is the special inyan of ha'erev shemesh?
Similarly, the Mishnah in Brachos (1:1) asks: "From when may one begin reciting Krias Shema at night?" The answer: "From the time the kohanim enter to eat their terumah." The Gemara clarifies that this means tzeis hakochavim (nightfall, when stars appear). Here too, it is puzzling: Why does the Mishnah base the time when one may begin reciting Krias Shema on when the kohanim eat terumah? Why not simply say "at nightfall"?
The Mishchas Shemen, quoting the introduction to the Iglei Tal, offers a beautiful insight:
When a person performs a mitzvah, that mitzvah leaves a positive, holy impression on him, uplifting and accompanying him throughout the day. Conversely, if one, chas v'shalom, commits an aveirah, the negative impression lingers with him all day. However, once the day ends and night begins, the spiritual influence of the mitzvah or aveirah departs. A new cycle begins.
Therefore, even after a kohen immerses in the mikvah, the residual impression of his prior tumah remains with him for the rest of that day. Only after ha'erev shemesh — at nightfall — when the old day ends, does the roshem of tumah dissipate, and he becomes fully tahor, permitted to eat terumah.
This same concept explains why Krias Shema is repeated at night. When a person says Krias Shema in the morning and accepts upon himself ol malchus shamayim, that acceptance remains with him throughout the day. He doesn't need to reaccept it at Mincha because the morning acceptance continues. But at nightfall, the impression of the morning Krias Shema ends — the influence of that kabalah departs and he must renew his kabalas ol malchus shamayim by reciting Shema again.
There is another deeper explanation as to why Krias Shema is linked to the kohanim. How does klal Yisrael become ready for kedusha (holiness) and kabalas ol malchus shamayim? It begins with the kohanim. When a kohen purifies himself and becomes tahor enough to eat terumah, he elevates all of klal Yisrael along with him. His personal taharah uplifts the entire nation. Therefore, the Mishnah says that the time for reciting Shema at night - the time for accepting ol malchus shamayim anew - is linked to when the kohanim begin eating terumah. Their purification prepares all of Klal Yisrael to be ready once again for kabbalas ol malchus shamayim.
The Sifsei Chaim in Moadim 1, Maamar 2 ("Teshuvah - Derech HaTeshuvah") relates the following story. Once, a young man who had lived his whole life far from Torah came to the Chofetz Chaim brokenhearted; he wished to do teshuvah and needed guidance. The Chofetz Chaim gave him encouragement, taught him how to begin keeping mitzvos, and told him to start davening and learning Torah every day. The young man followed everything faithfully. After a few weeks, he came back crying, "Rebbe, I did everything you told me! I daven, I learn, I do mitzvos — but I still feel the same! I don't feel the full taharah inside! Why don't I feel completely changed?"
The Chofetz Chaim smiled warmly and said: "When a garment becomes filthy, even after washing it, some stains don't come out right away. It needs to sit out, be washed again, and over time, the marks slowly fade. So too, when a person returns to Hashem, even if he does everything right, the roshem — the impression — of the past needs time to fully leave. You are on the right path. Keep going, and with time, you will feel your soul shining brighter and cleaner."
