Although Shaving and Haircuts are prohibited until Lag Ba’omer, when Lag Ba’omer falls out on Sunday, shaving and haircuts are permitted on Friday in the honor of Shabbos, according to the ‘Rama’. However, according to some, it is Assur to shave or take a haircut on Motzei Shabbos.
It would also not be permitted to do so on Thursday evening, if one has time on Friday, according to the Mishnah Berura.
As always, please check with your Rav for clarification. TLS.