This Halacha is an excerpt from our Sefer
Buy me here or on Amazon.com
It is accustomed to increase in joy on Lag BaOmer. It is a Mitzvah to rejoice the joy of Rashbi. Due to this, “One is to rejoice with all his heart and soul and make a day of feasting and joy on the 18th of Iyar, and sing praise to Hashem from the book of Tehillim, however he is not to enter into drunkenness and frivolity, Heaven forbid.” This joyous celebration applies even in the Diaspora, although is much more exorbitant in Eretz Yisrael.
The severity of mourning on this day-A message from the Rashbi retold by the Arizal:
Rav Avraham Halevi testified before me that he was accustomed to recite Nachem daily in Shemoneh Esrei, lamenting the destruction of the Temple. One year on Lag BaOmer when he was by the Rashbi and included the above prayer in his Shemoneh Esrei, the Arizal approached him with a stern message from the Rashbi. “Rebbe Shimon came to me and told me to ask you why you chose to say Nachem on the day of his Simcha? Rebbe Shimon said that due to this you yourself will unfortunately experience a reason for condolence in the near future.” And so it was that in that month Rebbe Avraham’s eldest son passed away.
 Admur 493:5; Rama 493:2; Maharil 157; Igros Hakodesh Alter Rebbe 1 p. 117
 Mishnes Chassidim Iyar
 Igros Kodesh Admur Hazaken p. 117
Other opinions: Some question why one should celebrate the death of Rebbe Shimon when in truth one should fast on the day of the passing of a Tzaddik. [See Chasam Sofer 233 and Toras Moshe Vayikra; Toras Moshe Vayikra; Shole Umeishiv Chamisha 39] For this reason the Chasam Sofer did not desire to move to Eretz Yisrael, in order so he is not forced to participate in the new holiday, as he referred to it. [Chasam Sofer 233] In addition, he claimed that the tragedies associated with the Tzfas earthquake in 1836 was due to their choosing to live in Tzfas near Rashbi and not in Yerushalayim. [Toras Moshe ibid] The Rebbe however answered this question by stating that since Rebbe Shimon himself asked for this day to be celebrated, therefore it differs from all other days of passing of Tzaddikim. [Likkutei Sichos 7:343]
 Implication of Mishnes Chassidim ibid and Igros Kodesh Admur Hazakein ibid that do not differentiate in this matter between Eretz Yisrael and Chutz La’aretz.
 See Minchas Elazar 4:60 “In Eretz Yisrael, and especially in Meron, they have meals and dance with music. In the Diaspora however, although a festive meal is held amongst the Chassidim and Rebbe’s, it is not customary to have dancing and music, and it is considered a strange event in our areas”
 Peri Eitz Chaim Sefiras HaOmer 7, brought in Ateres Zekeinim 493