People in their year of mourning for deceased relatives are generally not allowed to participate in celebrations and group meals with friends.

The Question About Lag B'Omer In Meron
Rav Shteinman was recently asked if mourners are allowed to go up to Meron for Lag B'Omer celebrations by the kever Rashb"i, by the grave of Rabbi Shimon bar Yochai. The question is whether the celebrations that happen in Meron are considered a hilula, festivities commemorating the death of a tzaddik, or if they are considered joyous celebrations like parties and group meals.

Rav Shteinman's Doubt
Rav Shteinman's initial response was that he is unaware of the nature of the festivities on Lag B'Omer in meron, however when he was there at a different time of the year the atmosphere was serious and appropriate. Rav Shach, he said, prohibited people from going to Meron on Lag B'Omer because of the inappropriate and immodest celebrations taking place there, however, Rav Shteinman suggested, today might be different because supposedly there are mehadrin routes to the grave that avoid the general, inappropriate celebrations.

Rav Shteinman's Investigation
Rav Shteinman committed to finding out what really happens in Meron so he could give a correct psak. Rav Shteinman investigated the routes up, he asked everybody who came to speak with him if they had been to Meron on Lag B'Omer and if they answered in the affirmative he would press them for details.

Rav Shteinman became convinced that there has been a change for the better in the situation in Meron on Lag B'Omer. While there still might be inappropriate celebrations, one can easily avoid them by using the newer mehadrin routes. However, he also found out that there is a band playing music all night long at the main bonfire. As well, along the way there are many booths, peddling different things, that also play loud music along the route, all creating a celebratory atmosphere.

Lag B'Omer in meron

Rav Shteinman's Psak
Rav Shteinman therefore paskened that one who is in mourning is not allowed to go to Meron on Lag B'Omer and be a part of the celebrations, as it definitely has the status of a "seudas m'rei'im" - a group party - due to the non-stop loud music and dancing. (source: Kikar)

I am impressed that Rav Shteinman did not just assume what he heard about ti was correct, but he went out of his way to investigate exactly what happens so that he could give an informed psak.

It does not bother me that he did not know already what goes on, as the concept of going en masse the way people go nowadays is really new, and is mostly from the chassidim and sefardim. The fact that today litvishe and others go as well is a sign of the influence the chassidic communities have had on the litvishe and general ashkenazic communities. Rav Shteinman did not know because going is not an issue in his crowd. As a matter of fact, many gedolim over the years have been against going to meron, most notably the Chasam Sofer.