Nowadays Lag Baomer has become this great day of celebration and hundreds of thousands of people go to Meron to the grave of Rav Shimon Bar Yochai (Rashbi) in Meron. However, the fact is that there are no early sources for these minhagim before the 1700s.
Traditionally, the sefirah period has been considered a time of mourning. The most well-known reason given by the rishonim is the mourning is due to the death of the 24,000 students of R. Akiva who died during this time of the year. Interesting enough, for some reason all of these mourning prohibitions are lifted on Lag Ba-Omer. If we look in the Tur, the Shulhan Arukh as well as the various early commentaries on them, the only reason we find is that the students of R. Akiva stopped dying on Lag Ba-Omer. There is nothing mentioned about Rashbi or Meron in any early sources.
The most well known explanation to the connection between Rashbi and Lag Ba-Omer is the claim that Rashbi died on that day, and he was one of the students of R. Akiva. However, this is quite strange that we would celebrate Rashbi’s death. We don’t celebrate the yarzheit of Avraham Avinu, Moshe Rabbeinu, David HaMelech, or any other great people with bonfires. Rather, halakha states the opposite, to fast on a yahrzeit, especially on those days that great people died.
More problematic is that neither Chazal nor any of the Rishonim mention Rashbi dying on Lag Ba-Omer. This was pointed out by the Chatam Sofer in his teshuvot (Y.D. 233) and because of this, he was very skeptical of the way Lag Ba-Omer is celebrated. In fact, the main source for Rashbi dying is R' Chaim Vital in the Pri Eitz Chaim, however, this is actually a printing mistake. The Pri Eitz Chaim actually wrote the Rashbi was שמח on Lag Baomer and the printers mistakenly turned the ח into a ת and wrote שמת., that he died on Lag Baomer.
Another "new" minhag is that of the upsherin which has also become connected to Lag Baomer and Rashbi. This is another minhag that has no basis in earlier sources. This idea is mentioned nowhere in the Rishonim or any early sources. Professor Sperber [Minhagei Yisrael 8: 13-30] documents how this actually comes from many completely outside ancient sources (e.g. non-Jewish sources).
It is amazing to me how the Charedim truly believe that they are traditionalists and are doing exactly what the Rishonim, Amoraim, Tannaim, etc. did when in fact many of the popular Charedi minhagim like Lag Baomer bonfires and Meron, upsherin, etc. are new inventions and have no basis in earlier sources.