Zevachim 100B

רבא אמר: אידי ואידי אחר חצות, ולא קשיא; כאן קודם ששחטו וזרקו עליו, כאן לאחר ששחטו וזרקו עליו.

Rava said there is a different resolution to the contradiction between the statements of R' Shimon. Both this baraita and that baraita are discussing cases where the relative died after midday on the 14th of Nisan, and still it is not difficult. Here, his relative died before the priests would have slaughtered the Paschal offering and sprinkled its blood on his account, and he may not send the offering. There, the relative died after the priests slaughtered the Paschal offering and sprinkled its blood on his account. Since acute mourning at night is by rabbinic law, it is suspended to allow him to consume an offering that was already sacrificed.

אמר ליה רב אדא בר מתנה לרבא: אחר ששחטו וזרקו את דמו ⁠– מה דהוה הוה! אמר ליה רבינא: אכילת פסחים מעכבא, מדרבה בר רב הונא. אמר ליה: ציית מאי דקאמר לך רבך.

Rav Adda bar Mattana said to Rava: In a case where one’s relative died after the priests slaughtered the Paschal offering and sprinkled its blood, why should he be permitted to partake of the Paschal offering? What was, was, and although the offering was sacrificed, isn’t he still an acute mourner by rabbinic law? Ravina said to Rav Adda bar Mattana: Partaking of the Paschal offering is indispensable for the mitzva, as is seen from that which Rabba bar Rav Huna taught, as the Talmud will explain. Therefore, the rabbis did not prohibit the acute mourner from partaking of the Paschal offering as they do for other offerings, for which consumption of the meat is dispensable. Rava said to Rav Adda bar Mattana: Listen to what your master, Ravina, told you, as his explanation is correct.

מאי דרבה בר רב הונא? דתניא: יום שמועה כיום קבורה למצות שבעה ושלשים; ולאכילת פסחים ⁠– כיום ליקוט עצמות. אחד זה ואחד זה ⁠– טובל ואוכל בקדשים לערב.

What is the statement of Rabba bar Rav Huna that teaches that partaking of the Paschal offering is indispensable? As it is taught in a baraita: The day that a person receives tidings that his relative died is considered as if it were the day of burial with regard to the mitzva of the 7-day mourning period, when he may not bathe or wear shoes, and with regard to the 30-day period when he may not wear ironed garments. And with regard to partaking of the Paschal offering, the day he receives the tidings is like the day of the gathering of the bones of the deceased after the flesh decomposed (see Pesaḥim 92a). In both this and that case, he immerses and partakes of sacrificial meat in the evening.