Lag BaOmer is the yahrzeit, Yom Hilula, of the Tanna HaKodosh, Rebbi Shimon bar Yochai (Rashbi), who is “Reb Shimon” in the Gemara and authored the Zohar HaKodosh. The Chida says we are joyous in honor of Rashbi, because that is what he wanted. (Avodas HaKodesh)
Why do we mourn on 7 Adar, the yahrzeit of Moshe Rabbeinu (many people used to fast on this day), but celebrate on the yahrzeit of Rashbi? Because the Gemara says (Temuros 16a) that 3000 halachos were forgotten during the mourning for Moshe Rabbeinu, therefore, we mourn. On the day Rashbi left this world, however, so man secrets of the Torah were revealed, making it a day of joy and celebration. (Ner Yisroel)
The days of Iyar are mesugal for refuah, especially the days of hod, and more so Lag BaOmer, which is hod sheb’hod.
The Arizal says that Rashbi blesses all those that come to Meron to celebrate on his Yom Hilula. (Hilula D’Rashbi)
The Tzemach Tzedek points out that the first letters of מ׳לך ר׳ופא נ׳אמן ו׳רחמן spell מרון, meaning in the merit of the tzadik at rest in Meron, Hashem will heal the sick.
The posuk (Yeshaya 57:19) says שָׁלוֹם שָׁלוֹם לָרָחוֹק וְלַקָּרוֹב, אָמַר ד׳-וּרְפָאתִיו – peace, peace to those far and those near, says Hashem, and I will heal him. ורפאתיו is the same gematria as שמעון בר יוחאי. The allusion from this is that even those who couldn’t come to Meron for Lag BaOmer can have their prayers answered, wherever they are (as the posuk says “to those far and those near”), and receive a yeshua in the merit of Rashbi. (Hilula D’Rashbi)
Similarly, Mesechta Pesachim (51b) says one can rely on Rashbi, if you’re in front of him or if you’re not in front of him. This alludes that one can receive a yeshua on his Yom Hilula “in front of him”, praying by his tzion, or “not in front of him”, meaning, any other place you may be on Lag BaOmer. (Kol Aryeh)
R' Avrohom of Kalisk would say, when he went to the mikvah on Lag BaOmer morning, that he accepts upon himself the kedushah of Yom Tov.
R’ Chaim Elazar of Munkach points out an allusion from the second mishna in Rosh Hashona. It says on Rosh Hashana all the people pass by Hashem “kibnei maron”. This can be read as on Rosh Hashana, passes by Hashem, the merits of all the people mibichinas Meron, where Rashbi and his son, Rebbi Elazar, are buried. Through this merit the world continues, as it says, the tzadik is the foundation of the world. (Shaar Yisaschor)
It is brought down that Lag BaOmer is also a day of salvation for everything. (Shaar Yisaschor)
The light of Kabalas HaTorah starts on Lag BaOmer, therefore, the most important part of the day is Torah. (Bnei Yisaschor) It is especially good to learn the story of Rashbi starting on Shabbos daf 33b (That is daf "lag"), or other Gemaras with Rashbi.
R’ Yaakov of Husyatin says that it is a day to pray for material and spiritual things, because this is the day the mon started to fall, opening up a pipe of parnasa, not just for that generation, but for all generations after. Each and every year there is a renewal of the light of this great and holy day.
Sfas Emes heard from his grandfather, the Chidushei HaRim, that on Lag BaOmer one can merit Yiras Hashem.
R’ Yehoshua of Kaminka said the following to the Divrei Yechezkel of Shiniva: Lag BaOmer is the “aspect” of Yom Kippur, because on Yom Kippur the avodah could only be done by Ahron, who was the aspect of hod. So too, Lag BaOmer, which is hod sheb’hod, one is able to receive good yeshuas, just like on Yom Kippur.
There is a custom to light bonfires in honor of Rashbi. The souls of all the tzadikim are there when the fire is lit.
R’ Avrohom Yaakov of Sadiger says tens of thousands of neshomas get tikunim from the lighting of bonfires in honor of Rashbi.
R’ Moshe Mordechai of Lelov says just saying over the name of the city, Miron, where Rashbi is buried, can bring yeshuas. This can be learned from the mishna in Yoma 3:1 which says they asked if the sky was lit up till Chevron. The Yerushalmi and Rav Ovadiah Bartenura explain Chevron is mentioned to bring about Zchus Avos, since the Avos are buried in Chevron. Similar to this, the Sadigerer said just saying the name of the city, Berdichev, brings rachamim from heaven, since R' Levi Yitzchok of Berdichev was always defending Klal Yisroel.
The Kedushas Zion of Bobov HY”D, in a letter told someone who wanted a child to donate Chai Rotel Mashke (about 54 liters of drink) for the people in Miron, in honor of Rashbi. (Taamei Minhagim) This is also done as a segula for many different things. See Tzidkas Rashbi to participate and send a kvitel. They have a video you can watch of Lag BaOmer in Miron and Chai Rotel in action.
There is a custom to put olive oil in the fire and to burn clothes in the fire. The Rizhiner used to send silk garments to Miron to be burnt in the bonfire. Tzadikim of Eretz Yisroel say this is a segula for success, materially and spiritually.
Reb Mendel of Rimanov says that the reason we use Bows and Arrows is because no rainbows were seen while Rashbi was alive and the bow symbolizes the rainbow. (Bnei Yisaschor)
A segula for one to have a boy is to shoot a bow and arrow on Lag BaOmer. (Sefer HaMidos) This is alluded from a posuk in Divrei HaYomim 1 8:40 וַיִּהְיוּ בְנֵי-אוּלָם אֲנָשִׁים גִּבּוֹרֵי-חַיִל דֹּרְכֵי קֶשֶׁת וּמַרְבִּים בָּנִים וּבְנֵי בָנִים And the sons of Ulam were mighty men of valor, shooters of bows (archers), and had many sons, and sons' sons. (Tcheriner Rov)
There is a minhag to cut a boys hair for the first time, if he is near the age of 3. There are many different customs related to this. See Nitei Gavriel who has a very thorough volume dedicated to this, listing every imaginable minhag of Upsherin.
Zohar & Tikunei Zohar
Many tzadikim say one should read Zohar or Tikunei Zohar, even if you don’t understand anything, because it is beneficial to the neshama. (This applies any day, but more so on Lag BaOmer) Among those that said this or similar statements to this are: Chida, R’ Yaakov Koppel, Degel Machane Ephraim, Shearis Yisroel, Komarna, Pela Yoetz, R’ Moshe of Kobrin, Shomrei Emunim and others.
R’ Pinchas of Koritz says reading Zohar is good for parnasa.
Many tzadikim say to read Tikunei Zohar, especially, from the beginning of Elul until Yom Kippur. (Sichos HaRan, Mateh Ephraim)
See A Simple Jew for Shem Mishmuel on Lag Baomer.