Shulchan Arukh, Yoreh De'ah 398

אבלות יום ראשון דאורייתא. ובו סעיף אחד:
אבלות יום ראשון אם הוא יום מיתה וקבורה הוי דאורייתא ושאר הימים מדרבנן במה דברים אמורים בשבעה מתים המפורשים בתורה שכהן מטמא להם אבל אותם שהוסיפו עליהם כדאיתא בסימן שע"ד אפי' ביום ראשון הם מדרבנן ויש אומרים שאף אבילות יום ראשון דמיתה וקבורה הוי דרבנן בכל המתים: [With respect to the observance of] mourning rites on the first day, — [the law is that] if it is the day of death and burial, it is regarded as Biblical and the remaining days [of mourning] are Rabbinical.¹The following authorities differ with respect to mourning on the first day: a) The Geonim, Maim. (Yad, Ebel I, 1), Alfasi (Ber. II; M.K. III), RaShBA, hold that the first day of mourning is Biblical, provided the burial is held on the same day as death, and the remaining days are Rabbinical. b) RaBaD adds to a) the thirty days forbidden to wear laundried garments and the prohibition against hair-cutting, as Biblical. T.H. rejects this. c) RI and R. Tam hold that no mourning is Biblical. Thus also R. Jonah in Ber. ibid. citing the French scholars. N in T.H. agrees with a), but maintains that it refers only to those near-of-kin mentioned explicitly in Scripture and not to those dead relations added by Rabbinical interpretation (v. Caro anon). Cf. also Zeb. 100a; Asheri M.K. ibid.; O.Ḥ. § 548, 3. further on the seven days of mourning, Gen. L, 10, Torah Temimah. This applies only to the seven dead [near-of-kin] explicitly mentioned in the Bible for whom a Kohen² Glos. may defile himself;³These are one’s a) father; b) mother; c) son; d) daughter; e) brother; f) unmarried sister; g) lawfully married wife. but those dead [near-of-kin] which were added to them [by Rabbinical exposition]⁴These are one’s a) maternal brother; b) married maternal sister or a betrothed maternal or paternal sister; c) outraged or seduced sister. Maim. (Yad, Ebel II, 1) includes the wife among those near-of-kin derived by Rabbinical exposition. On this N in T.H. writes that if the wife is of age, it is Biblical; if not of age, it is Rabbincal (v. however, B.Yos. a.l.). as stated in § 374, — even on the first day are regarded as Rabbinical [regarding the observance of mourning rites]. However, some say that even mourning on the first day [which is both the day] of death and burial is regarded as Rabbinical in all cases of dead [near-of-kin].⁵ supra n. 1c. This is the accepted ruling. infra § 399, 13 Gloss.