9. One who transgressed and did not keep Shiva:[1]

An Avel who did not keep Shiva is required to do so anytime within the Shloshim.[2] This applies whether he transgressed purposely or by mistake. [This applies even though he was aware of the passing and burial at the time that it occurred. This applies irrelevant of the relation that he shares with the deceased, whether he is the son, daughter, parent or sibling. Regarding whether if he is to perform Keriah-see Chapter 6 Halacha 1D. In the event that he kept Shiva after the official Shiva, he nevertheless keeps Shloshim from thirty days from the burial.]

If one kept some days of Aveilus but did not keep seven days:[3] The above only applies if one did not keep Shiva at all throughout all seven days of Shiva, if however, he observed some days of Shiva while other days he belittled [and did not observe Aveilus[4]], he is not required to make up those days. Thus, for example, if he did not observe the first day of Shiva, and began observing from day two, he nevertheless counts the seven days from day one.[5] Likewise, if one began sitting Shiva and for whatever reason stopped keeping Aveilus within Shiva, he is not required to make up the missed days after Shiva.[6]

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[1] Michaber 396:1; Tur in name of Raavad; Based on Moed Katan 20a; Pnei Baruch 25:1

If one was sick and did not keep Shiva and is now healthy within Shloshim-must he/she now keep Shiva: See Pnei Baruch 25:3 footnote 6-7 and Nitei Gavriel 113:18 footnote 35 [brought in Chapter 12 Halacha 3] for a dispute in Poskim of this matter. Practically, if the person kept some matters of Aveilus during the Shiva, it is valid and he:/she is not required to make it up. Regarding if the Regel is Mivatel Aveilus for such a person-see Poskim ibid.

[2] See Pnei Baruch ibid footnote 1; Tiferes Lemoshe 396

[3] Michaber 396:2; Tur 396 in name of Rosh Teshuvah 27; Shach 340:40 and 402:4; Radbaz 521, brought in Pischeiy Teshuvah 396:1; Pnei Baruch 25:2; See Gilyon Maharsha 396; Igros Moshe 3:155

Must one keep at least majority of the days of Shiva? See Pnei Baruch ibid footnote 4 for a dispute in Poskim of this matter. See Nitei Gavriel 113 footnote 33 in name of Poskim that even if he only kept Shiva for one moment it suffices.

Other opinions: Some Poskim rule that one must make up all the days of Shiva that he missed. [Implication of Ritva Moed Katan; Mordechai Aveilus 881; Raavad, brought in Rosh Moed Katan 3:25; See Igros Moshe Y.D. 3:158-2 that so is also the implication of the Kitzur SHU”A, Siddur Yaavetz and Derech Hachaim]

[4] See Pnei Baruch ibid and Nitei Gavriel ibid that some Poskim rule he must keep at least some Aveilus on those days that he belittled the Aveilus, and that he must have kept all the laws of Aveilus on the days that he did not belittle.

[5] Michaber and Poskim ibid

If the first day was also the day of death: Some Poskim rule that the day of death never counts as the first day of Shiva if Aveilus was not kept, as the first day is Biblical, and the above ruling in Michaber ibid is only regarding Yom Shemua. [Igros Moshe Y.D. 3:158-2]

Must one keep at least some laws of Shiva on the 1^st day: See Poskim in Halacha 3 who rule that one must keep at least some Shiva laws after the burial, before nightfall, for it to be considered the first day. This seemingly contradicts the simple understanding of Michaber ibid that the first day counts even if Aveilus was not kept. Perhaps, however, those Poskim only refer to one who attended the burial, as opposed to one who stayed home and is required to begin right away, or to a Shemua Kerova, in which case we rule that even if the first day was completely missed, it counts. Vetzaruch Iyun; See Igros Moshe ibid who implies that by Shemua Kerova it counts even if first day was completely missed.

[6] Shach 340:40 in name of Ramban and Tur although he concludes “This matter requires research”