Siman 396
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 אָבֵל שֶׁלֹּא נָהַג אֲבֵלוּתוֹ תּוֹךְ שִׁבְעָה, בֵּין בְּשׁוֹגֵג בֵּין בְּמֵזִיד, מַשְׁלִים אוֹתוֹ כָּל שְׁלֹשִׁים, חוּץ מֵהַקְרִיעָה, שֶׁאִם לֹא קָרַע בִּשְׁעַת חִמּוּם, אֵינוֹ קוֹרֵעַ, אֶלָא תּוֹךְ שִׁבְעָה, חוּץ מֵאָבִיו וְאִמּוֹ שֶׁקוֹרֵעַ אֲפִלוּ לְאַחַר שִׁבְעָה.
1 A mourner who did not observe his mourning rites during the seven [days of mourning], whether inadvertently or wilfully, may make up for it during the entire thirty [days of mourning] save for the rending of garments [concerning which the law is] that if he did not rend garments at the moment of excitement, he may rend only during the seven days [of mourning] save [in the case of] his father or mother [the law is] that he may rend even after the seven days [of mourning].
ב בַּמֶּה דְּבָרִים אֲמוּרִים, בְּשֶׁלֹּא נָהַג אֲבֵלוּת כְּלָל כָּל שִׁבְעָה; אֲבָל אִם זִלְזֵל בְּמִקְצָת הַיָּמִים, וְלֹא נָהַג בָּהֶם אֲבֵלוּת, כְּגוֹן מִי שֶׁאָמְרוּ לוֹ שֶׁמֵּת לוֹ מֵת בַּיּוֹם הָא', וְהָיָה בְּעִיר אַחֶרֶת וְלֹא נָהַג אֲבֵלוּת, וּבַיּוֹם הַשֵּׁנִי בָּא לְעִירוֹ וְנָהַג אֲבֵלוּת, אֵין צָרִיךְ לְהַשְׁלִים, וּמוֹנֶה שִׁבְעָה מִיּוֹם הָרִאשׁוֹן.
2 This applies only if he did not observe mourning rites at all for the entire seven [days of mourning], but if he was neglectful in [the observance of] some days during which he did not observe mourning rites, e.g., one who was informed on the first day that he suffered a bereavement, and [at the time this was reported to him] he was in another town and did not observe mourning rites, and on the second day he arrived in his own town and observed mourning rites, — [the law is that] he is not required to complete [the days of mourning that he missed], but counts the seven days from the first day.
ג קָטָן שֶׁמֵּת אָבִיו וְאִמּוֹ, אֲפִלוּ הִגְדִּיל תּוֹךְ שִׁבְעָה, בָּטֵל מִמֶּנּוּ כָּל דִּין אֲבֵלוּת וְאֵינוֹ חַיָּב בּוֹ.
3 A minor whose father or/and mother died, even if he became of age within the seven [days of mourning], — the entire law of mourning lapses from him and he is not obligated therein.