Question: If a bris falls out on a Sunday during the mourning time of the Omer, the Chasam Sofer says one cannot shave Erev Shabbos but if Lag B’omer falls out on Sunday then the Rema says one can shave on Erev Shabbos. What is the difference between a bris and Lag B’omer?
A. The Rema (Orach Chaim 493:2) says that one should not cut his hair until Lag B’omer itself, but not by evening, however if it falls out on Sunday there is a custom to cut hair on Friday in honor of Shabbos. And one who is making a bris for his son can cut his hair during sefira in honor of the bris. The Mishna Berura (13) adds that the father, sandek and mohel can get a haircut the night before he goes to shul between mincha and maariv. If the bris is on Shabbos he is permitted to get a haircut the day before even before chatzos, midday.
Answer: See Dirshu footnote 27 that the difference between a bris and Lag B’omer is that a bris is only a yom tov, joyous occasion for the individual whereas Lag B’omer is a yom tov for everyone.