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Some are accustomed not to visit cemeteries throughout the month of Nissan. If one has the Yartzite of a parent, or other close relative, in the month of Nissan, he is to visit the grave on Erev Rosh Chodesh Nissan. Others however are accustomed to visit cemeteries during the month of Nissan, just like any other time of the year, and hence of they have a Yartzite, end of Shiva or Shloshim during Nissan, they visit the grave as usual. Nonetheless, in all cases, one is to avoid crying.
Kivrei Tzaddikim: There is no need to avoid visiting the gravesites of Tzaddikim during the month of Nissan.
May a Matzeiva [tombstone] be established during the month of Nissan?
 Nitei Gavriel Pesach 3:8 [p. 54]; See Piskeiy Teshuvos 429:4
 See Gesher Hachaim 1 p. 259; Teshuvos Vehanhagos 2:65; Nitei Gavriel ibid;
 Gesher Hachaim 1 p. 259; Orchos Rabbeinu 2:305
 Dvar Yehoshua 2:80; Darkei Chaim Veshalom 783; Piskeiy Teshuvos ibid; The Rebbe was accustomed to visit the Tziyon of the Rebbe Rayatz in the month of Nissan on various occasions.
[Otzer Minhagei Chabad 15 p. 13]
 See Nitei Gavriel 16:3; Piskeiy Teshuvos 429 footnote 17; 697:1
 Minchas Yitzchak 3:51 based on Levushei Mordechai Y.D. 2:140
 Kinyan Torah 2:122