Siman 398
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א אֲבֵלוּת יוֹם רִאשׁוֹן, אִם הוּא יוֹם מִיתָה וּקְבוּרָה, הָוֵי דְּאוֹרַיְתָא; וּשְׁאָר הַיָּמִים, מִדְּרַבָּנָן. בַּמֶּה דְּבָרִים אֲמוּרִים, בְּשִׁבְעָה מֵתִים הַמְּפֹרָשִׁים בַּתּוֹרָה שֶׁכֹּהֵן מִטָּמֵא לָהֶם, אֲבָל אוֹתָם שֶׁהוֹסִיפוּ עֲלֵיהֶם כִּדְאִיתָא בְּסִימָן שע"ד, אֲפִלוּ בְּיוֹם רִאשׁוֹן הֵם מִדְּרַבָּנָן. וְיֵשׁ אוֹמְרִים שֶׁאַף אֲבֵלוּת יוֹם רִאשׁוֹן דְּמִיתָה וּקְבוּרָה הָוֵי דְּרַבָּנָן בְּכָל הַמֵּתִים.
1 [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. This applies only to the seven dead [near-of-kin] explicitly mentioned in the Bible for whom a Kohen may defile himself; but those dead [near-of-kin] which were added to them [by Rabbinical exposition] 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].