Q. (Re- question 2664 above, on an operation to be performed on Lag Baomer). If he is advised to be operated on Lag Baomer, can he take a home haircut and shave on the night (Monday) of that day?
A. On question 161 – regarding the earliest time one can cut his hair on Lag Baomer. we wrote: “Remah (O.H. 493: 2) rules that one should wait until the morning of Lag Baomer day for having a hair cut. Mishna Berura  (ibid. 10) explains that it refers to after the netz. However, he points out (ibid. 11) that there are Poiskim who permit a haircut on the night before, from Shaar Hatziun (ibid. 12) it seems that he does not agree with them.
Nitey 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.”
The Rov maintains that in our case he should wait if possible and cut his hair during the day. In need he may do so after alos hashachar (daybreak).
Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit’a