Q. Can one make a Lag Baomer party (with a siyum) this year Wednesday night, (Lag Baomer is Thursday), after the shekia, with music?
A. Poskim disagree if marriages are permitted on the eve of Lag Baomer. Some maintain that the simcha applies only to the day itself and since Halacha often dictates that "miktzaz hayom kekulo" or part of the day counts as a complete day, they can be performed from the morning of Lag Baomer, but not on the prior evening. Others maintain that weddings are permitted on the eve also (See Chasam Sofer, O.H.: 142, Minchas Yitzchok 4: 84, Igrois Moshe 1: 159, on others).
On question 161 in regard to the earliest time one can cut hair on Lag Baomer, and if it can be done erev Lag Baomer after already reciting maariv, following the plag hamincha, we wrote: "Remah (O.H. 493: 2) rules that one should wait until the morning of Lag Baomer day for having a haircut. Mishna Berura (ibid. 10) explains that it refers to after the netz. However, he points out (ibid. 11) that there are Poskim who permit a haircut on the night before, from Shaar Hatziun (ibid. 12) it seems that he does not agree with them.
Nitei Gavriel (Pesach 3, 49 n. 21) explains that the above depends on the minhag of reciting or not reciting tachanun during mincha erev Lag Baomer.
Horav Shlomo Miller’s Shlit”a opinion is that even for those who permit hair-cutting on the night before, they definitely not include the time after the plag before nightfall, despite having prayed maariv after the plag"
In our case: Horav Shlomo Miller's Shlit'a opinion is that if one maintains that minhag, one can celebrate with music after nightfall and in need from after sunset.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit'a