Pesachim 92A

והמלקט לו עצמות ⁠— טובל ואוכל בקדשים.

And one who gathers the bones of his parents, who are buried in a temporary location for their flesh to decay and who is moving them to a permanent burial place must also observe a day of acute mourning by rabbinic decree. These mourners immerse and eat all types of sacrificial food at night. Since in these cases, even during the day, the mourning is by rabbinic decree, the rabbis did not extend it into the evening.

גר שנתגייר בערב פסח, בית שמאי אומרים: טובל ואוכל את פסחו לערב, ובית הלל אומרים: הפורש מן הערלה כפורש מן הקבר.

With regard to a convert who converted on Passover eve, Beit Shammai say: He immerses and eats his Paschal lamb in the evening. And Beit Hillel say: One who separates from the foreskin by being circumcised is ritually impure, like one who separates from the grave after coming in contact with a corpse. Consequently, he must first observe the 7-day purification process necessary to remove ritual impurity imparted by a corpse. Only then, from the 8th day onward, may he partake of sacrificial meat.

גמ׳ מאי טעמא? קא סבר אנינות דלילה דרבנן, וגבי פסח לא העמידו דבריהם במקום כרת, גבי קדשים העמידו דבריהם במקום עשה.

Talmud: What is the reason that an acute mourner may eat the Paschal lamb in the evening? The tanna of the Mishnah holds that the observance of acute mourning at night after the day of one’s relative’s death is a rabbinic prohibition. And with regard to the Paschal lamb, the rabbis waived their prohibition because they did not uphold their statement prohibiting consumption of sacrificial food in a situation in which doing so would violate a prohibition that carries the punishment of karet, as is the case with one who neglects to offer the Paschal lamb. On the other hand, with regard to other sacrificial food, they maintained the prohibition, because they upheld their statement in a situation in which neglecting to eat the sacrificial food entails only the neglect of a positive mitzva.