This coming Sunday, the 18th of Iyar, will be Lag Ba’omer. One does not say Tachanun on Lag Ba’omer (or by Minchah on Erev Lag Ba’omer). Some also do not say Lamenatzeach or have aveilim daven for the amud (Shulchan Aruch with Mishnah Berurah 493:2, Piskei Teshuvos 493:9).
According to the Rama, when Lag Ba’omer falls on Sunday, one may take a haircut on Friday lekavod Shabbos (because if one were to enter into Shabbos having not shaved and then were to shave on Sunday morning, it would disgrace the honor of Shabbos.
Sefardim do not follow this minhag and wait to shave or take a haircut until Monday morning (Shulchan Aruch with Mishnah Berurah 493:2 MB11).
Note: If one shaves on Friday morning and then shaves again on Friday afternoon, one seemingly indicates by the second shaving that the first was not lekavod Shabbos. Therefore, one should only shave once on Erev Shabbos in this scenario.