When between Pesach and Shavuos is one to practice the mourning customs?
One is required to practice the mourning customs for 33 days between Pesach and Shavuos. A variety of customs and opinions exist regarding the exact dates of which the mourning customs are to be practiced between Pesach and Shavuos.
First Opinion-From 1st of Omer until Lag BaOmer: Some Poskim rule that these 33 days begin from the first day of Sefira and continue until Lag BaOmer. [Practically so is followed by many Ashkenazim. The Sefaradim keep the mourning custom from after Pesach until the morning of the 34th day of the Omer.]
Third opinion- From Rosh Chodesh Iyar until the 3rd of Sivan: Some are accustomed to begin to practice the mourning customs from Rosh Chodesh Iyar [the 30th of Nissan] until three days prior to Shavuos [Shloshes Yimei Hagbala, which is the 3rd day of Sivan]. They conclude the mourning customs beginning from after daybreak of the 3rd of Sivan. [See footnote for a fourth custom mentioned in Poskim]
Choosing a custom: Practically, one must abide by the custom of his community and may not swerve to follow a different custom. This applies whether one desires to be more lenient or to be more stringent than his community’s custom. Nevertheless one may choose to be stringent upon himself also like the other opinions simply in order to suspect also for their opinion. If there is no custom in one’s area, or if there is doubt as to the custom of one’s community, then one may choose which ever custom he desires. One may also choose to be stringent like all the customs and thus keep the mourning customs from Pesach until Erev Shavuos, or from Pesach until the 3rd of Sivan, not including Lag BaOmer. Nevertheless, there is no obligation to do so. However one may not choose to be lenient like the leniencies of all the customs combined.
The Chabad custom: The Chabad custom is to be stringent like all the opinions and practice the mourning customs throughout the entire period between Pesach and Shavuos, beginning from Pesach and concluding Erev Shavuos.
May one who does not have a set mourning custom choose to follow one custom one year and another custom a different year?
Do the mourning customs apply during Chol Hamoed Pesach according to those who mourn until Lag BaOmer?
Some Poskim rule that the mourning custom apply during Chol Hamoed, and hence one may not dance or listen to music. However other Poskim rule that one may dance and hear music during Chol Hamoed Pesach and so is the accustomed practice.
 Admur 493/5-7
 Admur 493/5
Other opinions: Some Poskim rule one is only required to keep 32 days of mourning. [brought in Admur ibid in parentheses; See Darkei Moshe 493/3; Levush 493/2] Other opinions rule one is required to keep 34 days. [Michaber 493/2]
 First opinion in Admur 493/5; Michaber 493/2; Hamanhig 106 in name of Razah; Rabbeinu Yerucham 5/4 44; Tashbatz 1/178
 The reason: There are those that say that the 33 days extend until Lag BaOmer because on that day the last of the 24,000 students of Rebbe Akiva died. [Admur ibid; Michaber ibid] However according to the Michaber 493/2 the last of the students died on the 34th day of the Omer. [M”B 493/7]
When by Lag BaOmer does the mourning end according to this opinion? The mourning customs are followed during the night of Lag BaOmer up until after day break being that Lag BaOmer is the 33rd day of mourning, and even on this day there were some students who died [and hence reached the grand total of 24,000 deaths]. The mourning customs end after daybreak of Lag BaOmer, being we always apply the rule of Miktzas Hayom Kikulo to the last day of mourning, as explained in Yoreh Deah 395/1. According to this opinion, from that time and onwards the mourning customs are no longer followed. [Admur ibid; Rama 493/2] However there are areas that are accustomed to end the mourning customs immediately by the entrance of the night of Lag BaOmer. (Their reason is because in their opinion one is only required to practice 32 days of mourning and not 33.) [Admur ibid in parentheses; See Darkei Moshe 493/3] This however is only in accordance to the ruling of the Rama ibid that the last of the students stopped dying on the 33rd day of the Omer. However according to the Michaber 493/2 the mourning custom fully apply up until the morning of the 34th day of the Omer as in his opinion the last of the students died on the 34th day of the Omer.
 However during the night of the 34th and the entire 33rd [Lag BaOmer] all the mourning customs remain active. [Michaber 493/2 as explained in M”B 493/6]
 Michaber 493/2; Peri Chadash 493/1; Mamar Mordechai 493/3; Kaf Hachaim 493/25; Yabia Omer 3/26; Minchas Yitzchak 4/84 that so is the Sefardi custom
 Brought in Admur 493/6; mentioned in Michaber/Rama 493/3; Maharil p. 157; The Michaber ibid states “Those that take a haircut on Rosh Chodesh Iyar are mistaken.” The Rama ibid however defends their custom, stating that so is the custom in many places.
 The calculation of 33 days according to this opinion: Although on Lag BaOmer the mourning customs are suspended, nevertheless since they are only suspended beginning from after the morning of Lag BaOmer, in which we apply the rule of Miktzas Hayom Kikulo, therefore the 33rd day of the Omer is included in the calculation of the 33 days in which we are accustomed to mourn. [Admur ibid] The calculation is as follows: 28 days in Iyar [including Lag BaOmer], plus 5 days in Sivan, until the morning of the 5th of Sivan [Erev Shavuos], for a total of 33 days.
 Brought in Admur 493/6; M”A 493/5 “so is the custom in his country”
 Machatzis Hashekel on M”A ibid; Chayeh Adam 131/11; Kaf Hachaim 493/25; Admur and M”A ibid do not write until which day of Rosh Chodesh this custom begins the mourning, the first which is 30th of Nissan or the 2nd day of Rosh Chodesh which is the 1st of Iyar. Nevertheless in order to have a calculation of 33 days one must state it refers to the first day of Rosh Chodesh Iyar which is the 30th of Nissan.
 Kaf Hachaim 493/25
 The calculation of 33 days according to this opinion: This custom is based on the premises held of by the 2nd opinion that one can consider the partial mourning of a day as a full day [even in the middle of the mourning]. They end the mourning starting from after daybreak of the first of the Shloshes Yimei Hagbala and as a result they start their mourning period on the [first day] of Rosh Chodesh Iyar [which is the 30th of Nissan, as opposed to the 2nd day of Iyar as followed by the 2nd opinion] This is done in order to have a total of 33 days of mourning. [Admur ibid] One day of Nissan, the entire month of Iyar [29 days] and 3 days in Sivan, until the after daybreak of the 3rd of Sivan, hence having the 3rd of Sivan to follow the rule of Miktzas Hayom Kekulo.
 Fourth custom: Some Poskim rule that one is to keep the laws of mourning for the entire period of time, before and after Lag BaOmer, and only on Lag BaOmer may one take a haircut. [Mateh Moshe 688; Levush; Elya Raba 493/6]
 Admur 493/7
 Admur ibid; Rama 493/3
 Admur ibid
The reason: It is forbidden to swerve from the community custom, whether to be lenient or stringent, due to the prohibition of “Lo Sisgodedu”, which is a prohibition against making sects which make the Torah appear like two different religions, as he is being stringent and they are being lenient, or the opposite. [Admur ibid; Rama ibid] This applies even if there is no worry that doing so will cause strife in the community, as there are many people in the community that follow one custom and many that follow another custom, and they are hence not particular on each other. [Admur ibid]
 When being stringent in a place that the custom is to be lenient one is not allowed to be stringent in a way that he shows that he is doing so because he thinks the stringent opinion is the main Halachic opinion that everyone should follow, as this enters into the prohibition of Lo Sisgodedu. However one is allowed to be stringent upon himself simply in order to suspect for the stringent opinion, and not as a way of obligation. [Admur ibid]
 Admur ibid
The reason: We do not suspect that perhaps the custom of his community is different than the custom which he has chosen being that mourning is a mere custom and hence one is not to be stringent in a case of doubt. [Admur ibid]
 The reason: Although by doing so one is following two contradictory stringencies, nevertheless one is not considered like an ignorant fool, as he is doing so as a result of his question as to which custom is the main custom. [Admur ibid]
 Admur ibid; Chok Yaakov 493/11
 Admur ibid; Rama ibid
Meaning one may not delay the mourning customs until Rosh Chodesh Iyar as rules the second opinion, and also end the mourning customs after Lag BaOmer and onwards until Erev Shavuos, as rules the first opinion, as these are two contradictory leniencies. [Admur ibid]
 This custom follows the option given by Admur 493/6 to be stringent like all the customs; This Chabad custom is also evident from the Igros Kodesh 8/318 and 9/106 in which the Rebbe writes that we do not make weddings until after Shavuos. See also Likkutei Sichos 37 p. 123 footnote 8 “Although some are accustomed to permit haircuts and weddings after Lag Baomer, nevertheless the Chabad custom is to prohibit also after Lag BaOmer, as rules Admur in 493/6” I have not found any explicit source for the Chabad custom other than the above letters and Sicha of the Rebbe [which do not state when the mourning begins]. The Rebbe Rashab discouraged taking a haircut during the three days before Shavuos, until Erev Shavuos. However this is based on the Arizal and is not relevant to the Chabad custom of mourning. See Nitei Gavriel 48/7 that the widespread custom of Ashkenazim is to not get married until Rosh Chodesh Sivan. In the later years the Rebbe allowed weddings beginning from the three days of Hagbala. Based on this some in Chabad are lenient today to end all the mourning customs by that time. The Rebbe Rashab discouraged taking a haircut during the three days before Shavuos, until Erev Shavuos.
 Chasam Sofer 142; Minchas Yitzchak 4/84
 P”M 493 M”Z 2
 See Az Nidbaru 10/23; Mishneh Halachos 8/188