It is not difficult. Here, in the baraita where Rabbi Shimon holds an acute mourner may not send a Paschal offering, since acute mourning at night is by Torah law, it is referring to a case where his relative died on the fourteenth day of Nisan and he buried him on the fourteenth itself. There, the ruling in the mishna in tractate Pesaḥim, which teaches that an acute mourner immerses and partakes of the Paschal offering in the evening, since acute mourning at night is by rabbinic law, is referring to a case where his relative died on the thirteenth day of Nisan and he buried him on the fourteenth of Nisan.

Rav Mari explains: In a case where his relative died on the fourteenth day of Nisan and he buried him on the fourteenth itself, his acute mourning is due to the day of death and is therefore by Torah law. Consequently, it takes hold of its following night by Torah law, and the mitzva of the Paschal offering does not override it. By contrast, in a case where his relative died on the thirteenth day of Nisan and he buried him on the fourteenth of Nisan, the fourteenth is only the day of burial, and his acute mourning is therefore by rabbinic law. Consequently, it takes hold of its following night only by rabbinic law, and the mitzva of the Paschal offering overrides it.

Rav Ashi said to Rav Mari: But if so, it is difficult to understand that which the baraita teaches: Rabbi Shimon said to Rabbi Yehuda: Know that this so, as the Sages said: An acute mourner immerses on the fourteenth of Nisan and partakes of his Paschal offering in the evening, but he may not partake of other sacrificial meat. According to your explanation of this statement, let Rabbi Yehuda say to Rabbi Shimon that this is no proof: I am telling you a halakha about the day of death, when acute mourning is by Torah law, and you tell me that you have a proof from a mishna that deals with the day of burial, when acute mourning is by rabbinic law. The Gemara concludes: This indeed poses a difficulty for Rav Mari.

Abaye said a different resolution to the contradiction between the statements of Rabbi Shimon: It is not difficult. Here, in the baraita where Rabbi Shimon holds that an acute mourner may not send a Paschal offering, it is referring to a case where his relative died before midday on the fourteenth of Nisan. There, the ruling in the mishna in tractate Pesaḥim, which teaches that an acute mourner immerses and partakes of the Paschal offering in the evening, it is a case where his relative died after midday on the fourteenth of Nisan. When his relative died before midday, in which case he was not ever fit for bringing a Paschal offering since the obligation begins at midday, the status of acute mourning applies to him, and it is prohibited for him to bring a Paschal offering. But if his relative dies after midday, when he is already fit for bringing a Paschal offering, the status of acute mourning does not apply to him with regard to this matter, so he may immerse and partake of the Paschal offering in the evening.

The Gemara explains: And from where do you say that the halakha is different depending on whether his relative died before midday on the fourteenth of Nisan or whether he died after midday? As it is taught in a baraita: The Torah states with regard to a priest: “And for his sister a virgin, that is near unto him, that has had no husband, for her may he become impure” (Leviticus 21:3). From here it is derived that it is a mitzva for a priest to become impure in order to bury his deceased relatives, and if he did not want to do so, others must render him impure against his will. And an incident occurred involving Yosef the priest, where his wife died on Passover eve, and he did not want to become impure, as he wanted to offer the Paschal offering; and his brethren the priests voted and rendered him impure against his will.

And the Sages raise a contradiction from another baraita: What is the meaning when the verse states with regard to a nazirite: “He shall not become impure for his father, or for his mother, for his brother, or for his sister when they die; because his consecration unto God is upon his head” (Numbers 6:7)? If a nazirite went to slaughter his Paschal offering or to circumcise his son, mitzvot for which failure to fulfill them is punishable by karet, and he heard that a relative of his died, one might have thought that he should become impure in order to bury his relative even if this will result in his not bringing the Paschal offering. You rather say, based on this verse, that “he shall not become impure.”

One might have thought that just as he may not become impure to bury his sister, so too he may not become impure to bury a corpse with no one to bury it [met mitzva]. The verse states: “Or for his sister,” teaching that it is only to bury his sister that he may not become impure, but he does become impure to bury a met mitzva.

In the first baraita, contracting impurity from a dead relative is obligatory, and in the second baraita it is forbidden. Do you not learn from this contradiction that there must be a distinction between the cases? Here, in the first baraita concerning a priest, it must be referring to a case where the relative died before midday, so he was obligated to become impure before the obligation for the Paschal offering took effect. There, in the second baraita concerning a nazirite, it must be referring to a case where the relative died after midday. This distinction, between death before and after midday, also applies to acute mourning.

The Gemara rejects this explanation: From where do you infer that this is the resolution to the contradiction? Actually, perhaps I will say to you: Both this baraita and that baraita are discussing deaths that occurred after midday. And this baraita, about a nazirite, is in accordance with the opinion of Rabbi Yishmael, and that baraita, about Yosef the priest, is in accordance with the opinion of Rabbi Akiva. As it is taught in a baraita: The Torah states with regard to a priest: “And for his sister a virgin, that is near unto him, that has had no husband, for her may he become impure” (Leviticus 21:3). This is optional, i.e., a priest is not obligated to become impure to bury his sister; this is the statement of Rabbi Yishmael. Rabbi Akiva says: It is mandatory.

The Gemara responds: This should not enter your mind, because it is Rabbi Akiva who teaches the first clause of the baraita about a nazirite. As it is taught in the full version of that baraita: The Torah states: “All the days that he consecrates himself unto the Lord he shall not come near to a dead body. He shall not become impure for his father, or for his mother, for his brother, or for his sister when they die; because his consecration unto God is upon his head” (Numbers 6:7). Rabbi Akiva says: The term “body [nefesh]” is referring to relatives. The term “dead [met]” is referring to non-relatives.

The verse specifies: “For his father,” even though his father is included among his relatives, to teach that he may not become impure to bury his father, but he does become impure to bury a met mitzva. The verse states: “For his mother,” to teach that even if he was a priest and he was a nazirite, and therefore doubly prohibited from becoming impure, nevertheless, he may not become impure to bury his mother but he does become impure to bury a met mitzva. The verse states: “For his brother,” to teach that even if he was a High Priest, who may not become impure even for his relatives, and he was also a nazirite, nevertheless, he may not become impure to bury his brother but he does become impure to bury a met mitzva.

What is the meaning when the verse states: “Or for his sister”? If a nazirite went to slaughter his Paschal offering or to circumcise his son, and he heard that a relative of his died, one might have thought that he should become impure. You rather say that he may not become impure. One might have thought that just as he may not become impure to bury his sister, so too he may not become impure to bury a met mitzva. The verse states: “Or for his sister,” to teach that he may not become impure to bury his sister but he does become impure to bury a met mitzva. Consequently, the statement of the baraita concerning a nazirite represents the opinion of Rabbi Akiva.