(Halachically Speaking-repost)
According to the opinion of the Mechaber, one may not take haircuts until the morning of the thirty-fourth day. The Rama holds that one may shave on Lag B’omer. Many apply this to the night of Lag B’omer (because tachanun is already omitted on erev Lag B’omer), while others say it means first thing in the morning. This is based on the concept that if one waits a little in the morning it is as if he waited a whole day (miktzas hayom k’kula). The custom of many seems to be that shaving or taking a haircut is permitted after neitz hachama.
When Lag B’omer falls out on Sunday (as it does this year), shaving and haircuts are permitted on Friday because of kovod Shabbos. The logic is that tachanun is already omitted on erev Lag B’omer, so technically shaving would permitted on Shabbos. Since one cannot shave on Shabbos, the allowance is pushed back to Friday. One may not shave on Motzei Shabbos when Lag B’omer falls out on Sunday. Furthermore, many say that shaving is not permitted on Thursday night if one has time to do it on Friday. One who started shaving on Lag B’omer may continue after shekia as well (even if he does not shave after Lag B’omer).
According to the opinion of the Arizal, shaving is prohibited even on Lag B’omer. Shaving is permitted on Erev Shavuos even for those who follow the Arizal’s opinion.
Read the complete article on all the halachos/ customs of Lag B’Omer here.
Ariel Baradarian says
It should be noted that according to Rav Ovadia Yosef and Rav Ben-Tzion Abba Shaul, even if one holds according to the Arizal, one can shave his beard even on Lag B’Omer. They reasoned that since the Arizal frowned up shaving the beard all year round, he did not specify the beard at all for the omer. In other words, the chumra of the Arizal only applies to the hair of the head, not of the beard.
However, I believe others, including Rav Mordechai Eliyahu disagree and hold that the beard is also part of the chumra.
See Yalkut Yosef Saka Edition on Sefirat HaOmer for more details.