Dear Rav Pfeffer shlita,
In light of the fact that the Rema only allows the laws of mourning to be lifted by the morning of Lag Baomer and the fact that the Mishnah Berurah brings that even according to those who permit one to cut hair on the night of Lag Baomer, weddings are forbidden. Now although many Achronim challenge that din, that’s because for the mitzvah of a wedding should be more meikel than cutting hair, but to have a party of dancing and music that so many people do on the night of Lag Baomer, what is the heiter for this?
Several poskim rule that the entire day of Lag Ba-Omer, including the night before, is a time of joy, and that the customary mourning does not apply during that time. See Chok Yaakov, Mor U-Ketziah, Maadanei Yom Tov, aomng others.
Based on this ruling, it is fine to have a Lag Ba-Omer party on the night of Lag Ba-Omer, and the special mitzvah of a wedding is not required as part of the leniency.
In addition, the party itself is a mitzvah of sorts, in that it celebrates the joyful day of Lag Ba-Omer. In fact, there is room to suggest that there is more reason to be lenient for a “Lag Ba-Omer party,” where the joy is part of the Omer period, than for a wedding which involves joy that it unrelated to the Sefirah.
As you note, the general practice is to be lenient.
Best wishes and kind regards.