Breaking Shiva/Shloshim:[1]  

The holiday of Pesach nullifies Shiva [or Shloshim]. Thus, if one began Shiva before Pesach for even a short amount of time, the Shiva is nullified by Pesach [i.e. by midday of Erev Pesach[2]] and counts as seven days towards the Shloshim. [Even if the burial took place on Erev Pesach after midday, one sits Shiva prior to Yom Tov, and at the entrance of Yom Tov his Shiva is nullified.[3] Some Poskim[4], however, rule that after midday of Erev Pesach, one is to only follow the private mourning laws that are followed on Shabbos, and it suffices to annul the Shiva.] The 7/8 days of Pesach [i.e. seven in Eretz Yisrael and eight in Diaspora] also count towards the Shloshim and hence after Pesach he is to count 15/16 days of Shloshim. [If one ended the Shiva before Pesach, then Pesach nullifies the Shloshim, as explained in Halacha 2A.]

Buried during Chol Hamoed:[5] If the burial took place during Chol Hamoed then the last day of Yom Tov does not break the Shiva.[6]

The status of Erev Pesach within Shiva:[7] On Erev Pesach, the Shiva is nullified at midday, and hence one becomes permitted in all matters of Shiva [i.e. leather shoes; learning Torah; sitting on chair] as soon as midday arrives.[8] [However, one must wait until it is near sunset in order to bathe, as explained above in Halacha 1B.[9] However, one may launder his clothing for Yom Tov anytime on Erev Yom Tov, as stated there.[10] Due to the Shloshim, one may not get a haircut, as stated there.]

Cutting nails on Erev Pesach within Shiva:[11] Some Poskim rule an Avel during Shiva may cut his nails on Erev Pesach in order to clean them from Chametz. It is to be cut by another person, or with a Shinui.

The status of Erev Pesach within Shloshim:[12] On Erev Pesach [the Shloshim is nullified at midday, and hence] one becomes permitted in all matters as soon as midday arrives.[13] Nevertheless, it is best to get a haircut prior to midday, being that it is forbidden for others to get a haircut after midday.[14] [Nonetheless, if one did not get a haircut prior to midday, he may do so after midday.[15]]

[1] Michaber O.C. 548:12; Y.D. 399:7

[2] Rama 399:3 based on Mahariy Mintz 64 regarding customs of Shloshim and the same would apply regarding Shiva; Tashbeitz; Chochmas Adam 169:3; Daas Torah 399:3 in name of Or Zarua; Mishmeres Shalom Ayin 20; Shevet Halevi 6:173; Nitei Gavriel 124:13

If buried Erev Pesach before midday: If one’s relative was buried on Erev Pesach before Chatzos, some Poskim rule that one sits Shiva until Chatzos, and starting from Chatzos one’s Shiva is nullified. [Rama 399:3 based on Mahariy Mintz 64 regarding customs of Shloshim and the same would apply regarding Shiva; Tashbeitz; Chochmas Adam 169:3; Daas Torah 399:3 in name of Or Zarua; Mishmeres Shalom Ayin 20; Shevet Halevi 6:173; Nitei Gavriel 124:13]

Other opinions: Some Poskim rule that even on Erev Pesach, the laws of Shiva apply close to sunset. [Shach 399:11 questions the ruling of Rama, and his understanding in the Mahariy Mintz ibid and concludes with a Tzaruch Iyun; See Birkeiy Yosef 399 based on Ritva; Kitzur SHU”A in Lechem Hapanim 220; Divrei Yatziv 240 that the ruling of Rama is limited to Shloshim, however, regarding Shiva, he too agrees that it is kept until night] Nitei Gavriel ibid concludes that we are lenient by Aveilus like the first opinion

[3] Implication of Michaber ibid and ibid that one hour before the night of Pesach suffices; All Poskim in next footnote

[4] Chochmas Adam 169:3; Mikraeiy Kodesh 58:35; Shevet Halevi 6:173; Based on Rama 399:3 regarding Shemua Kerova on Shabbos Erev Yom Tov; Nitei Gavriel Pesach 51:1; Aveilus 124:14; Vetzaruch Iyun from Setimas Hamichaber in O.C. and Y.D. ibid that implies one keeps full Aveilus before Pesach.

[5] So rule regarding Shemini Atazres nullifying Shloshim and the same applies regarding Shiva and the last day of Pesach: Michaber Y.D. 399:2; See Taz 399:4; Noda Beyehuda 14, brought in Pischeiy Teshuvah 399:8; See also Rav Akiva Eiger 399

[6] The reason: Although Shemini Atzeres is its own holiday and can potentially nullify Shloshim, this only applies if the Aveilus was already begun before the Holiday. Now although one has already begun some of the laws of Aveilus before Shemini Atzeres, such as not to launder and get haircuts, it does not suffice, as in order for Shemini Atzeres to nullify the Shloshim one must already have completed the Shiva. [Taz ibid] Thus, we see that Shemini Atzres was never considered a strong enough holiday to nullify Shiva, and its only power is to nullify Shloshim, and even this can only be done if Shiva was already completed beforehand. This certainly applies to the last day of Pesach which is not considered its own Holiday even regarding Shloshim.

[7] Rama 399:3 based on Mahariy Mintz 64 regarding customs of Shloshim and the same would apply regarding Shiva; Tashbeitz; Chochmas Adam 169:3; Daas Torah 399:3 in name of Or Zarua; Mishmeres Shalom Ayin 20; Shevet Halevi 6:173; Nitei Gavriel 124:13

Other opinions: Some Poskim rule that even on Erev Pesach, the laws of Shiva apply close to sunset. [Shach 399:11 questions the ruling of Rama, and his understanding in the Mahariy Mintz ibid and concludes with a Tzaruch Iyun; See Birkeiy Yosef 399 based on Ritva; Kitzur SHU”A in Lechem Hapanim 220:4 in length negates the understanding of Chochmas Adam ibid in opinion of Rama; Divrei Yatziv 240 that the ruling of Rama is limited to Shloshim, however, regarding Shiva, he too agrees that it is kept until night] Nitei Gavriel ibid concludes that we are lenient by Aveilus like the first opinion

[8] The reason: As at midday we began slaughtering the Pesach sacrifice [Rama ibid] and it is hence considered like a Holiday to the point that we no longer do Melacha. [See Shach 399:12]

[9] The reason: As the laws of Shloshim still apply, and we only revoke it for the sake of Yom Tov if it is near Yom Tov, as stated above.

[10] When on Erev Pesach may/should one launder? The Michaber 399:5 and O.C. 548:10 rules that laundering should be done after midday of Erev Yom Tov, however, perhaps on Erev Pesach it can be done before midday, being that there is a prohibition of Melacha after midday and everyone knows that one is laundering before midday for the sake of the Yom Tov. Nonetheless, we would not apply the rule that it must be done before midday [due to the Issur Melacha and ruling of Rama ibid regarding Shloshim to get a haircut before midday] as by a haircut the reason is because other people are prohibited in giving haircuts, and not because the Avel cannot do this Melacha after midday, as explains Shach 399:12 in negation of Levush. Thus, here regarding laundering where this reason of a haircut does not apply, it would seem that there is no preference of before or after midday. Vetzaruch Iyun Lemaaseh, as doing laundry after midday breaks the Issur Melacha [even though here we can permit an Avel] but makes it more recognizable, while before midday avoids the Issur Melacha, but makes it less recognizable.

[11] Nitei Gavriel 124:11; 2 7:8 in name of Chaim Bayad 125:76; Sdei Chemed 64; Darkei Chaim 13:8

[12] Rama ibid; Elya Raba 548:9; Chochmas Adam 169:3; Kitzur SHU”A 220:4; M”B 548:31

[13] Rama ibid based on Mahariy Mintz 64

The reason: As at midday we began slaughtering the Pesach sacrifice [Rama ibid] and it is hence considered like a Holiday to the point that we no longer do Melacha. [See Shach 399:12]

Other opinions: Some Poskim rule that even on Erev Pesach, one may not launder or bathe unless it is close to sunset, and it is only haircuts which is permitted prior to midday. [Shach 399:11 questions the ruling of Rama, and his understanding in the Mahariy Mintz ibid and concludes with a Tzaruch Iyun; See Birkeiy Yosef 399 based on Ritva]

[14] Rama 399:3; Mahariy Mintz ibid; Shach 399:12; Chochmas Adam 169:3; Kitzur SHU”A 220:4; ; M”B 548:31; Nitei Gavriel Vol. 2 5:7

The reason: As one may no longer do Melacha after midday, including haircuts. Now, although he may get a haircut after midday [as is the ruling even on Chol Hamoed regarding one who got up from mourning-Michaber 531:7] nonetheless, he will be forced to give himself the haircut as other people are forbidden in doing so for him. [Shach ibid; Mahariy Mintz ibid] Alternatively, the reason is because it is forbidden also for him to cut his hair after midday, just as is forbidden for others. [Levush 399, recorded, and negated, in Shach ibid]

[15] Implication of Rama ibid; Shach 399:12

Other opinions: Some Poskim rule that if one did not get a haircut before midday, he may not do so later on. [Levush 399, recorded, and negated, in Shach ibid]