Negaim, Chapter 3, Mishnah 3. This chapter deals with the people involved in the laws of nega'im: who is subject to this tumah, who is qualified to examine the mark, and how and when the inspection takes place. Our Mishnah teaches a remarkable halacha of timing. There are moments in a person's life when the kohen simply does not come to look, because the Torah itself did not wish his joy to be spoiled.
"Chasan shenir'ah bo nega, notnin lo shivas yemei hamishteh." When a bridegroom notices a nega on himself during the week of his marriage celebration, he is given the seven days of feasting. Those seven days carry for him the status of a festival, and therefore no kohen comes to inspect the mark while they last.
The Rav brings the source from the verse. The Torah writes "uveyom hero'os bo," on the day it is shown to him. Scripture could simply have written "uvehero'os bo," when it is shown to him. Why the addition of the word "day"? To teach that there is a day on which you do examine, and there is a day on which you do not examine. One of those days on which the kohen does not look is when a man has just been married and discovers a blotch on his skin. The kohen may not examine it, because that examination would ruin his seven days of rejoicing.
"Lo ul'veiso ulichsuso." The dispensation extends beyond the chasan's own body. If a nega appeared in his house, we do not examine it for the laws of nig'ei batim, and if a nega appeared on his garment, we do not examine that either, throughout his shivas yemei hamishteh.
"V'chein baregel, notnin lo kol yemos haregel." The identical consideration applies to Yom Tov, and there it applies to everyone, not only to a chasan. We do not wish to spoil a person's Yom Tov. If a spot appears at the beginning of the festival, the kohen does not come until the festival has passed, whether the spot is on his body, on his clothing, or in his house.
The Mishnah thus teaches us something about the character of these laws. As long as the kohen has not pronounced his ruling, the tumah has not taken hold, and the Torah gives room for a person's simchah, allowing the examination to wait until his days of celebration are behind him.