Many people observe the custom to refrain from shaving (in addition to refraining from haircutting) during the Omer period, until Lag Ba’omer. It is customary among Ashkenazim to permit shaving on the Friday before Lag Ba’omer when Lag Ba’omer falls on Sunday, out of respect for Shabbat. Sepharadim, however, do not follow this practice. The reason for this practice is because the custom of the Sepharadim is not to allow haircutting or shaving until the morning of the 34th day of the Omer – meaning, the morning after Lag Ba’omer. In a year when Lag Ba’omer falls on Sunday, then, Sepharadim may not shave or take a haircut until Monday morning. As such, there is no basis for permitting shaving on Friday. The provision allowing shaving on the Friday before Lag Ba’omer applies only to those who observe the custom to allow shaving on Lag Ba’omer itself; according to that custom, shaving is allowed on Friday if Lag Ba’omer falls on Sunday. According to the Sephardic custom, however, which does not allow shaving until Monday, there is no basis to permit shaving three days earlier, on Friday.
Shaving and haircutting are allowed on Monday morning, immediately after reciting the Shaharit service. One does not have to wait until the end of the day, due to the Halachic principle of "Miksat Ha’yom Ke’kulo" ("part of the day is like the entire day").
It is commonly explained that Lag Ba’omer is observed as a day of celebration because it marks the anniversary of the death of Rabbi Shimon Bar Yohai, and on that day his home was filled with a special holy fire. Just before his death, Rabbi Shimon revealed many secrets of Torah and Kabbala, which is compared to fire, and thus his coffin was surrounded by a spiritual fire. It is therefore customary to light bonfires on Lag Ba’omer, since Rabbi Shimon’s death occurred amid fire. Some have the custom to throw clothing into the bonfires to commemorate Rabbi Shimon’s stature of piety and holiness which resembled that of Adam before the sin, when he did not need clothing. There is also a time-honored custom to visit Rabbi Shimon’s gravesite on Mount Meron in Northern Israel, and to study Torah and passages from the Zohar at the site. It is documented that the Arizal would visit Meron with his students on Lag Ba’omer, and that they would bring their three-year-old sons to give them their first haircut at the holy site. Lag Ba’omer is also an especially precipitous time for having our prayers answered, and an occasion for Torah learning and drawing closer to the Almighty.
It might seem peculiar, at first glance, that we commemorate the death of a righteous person through festive celebration. One of the reasons given for this custom is that Halacha generally does not follow Rabbi Shimon’s rulings in the Talmud. It is only Bi’sh’at Ha’dahak, meaning, under extenuating circumstances, that we follow Rabbi Shimon’s opinions; ordinarily, we do not follow his view. Similarly, as a rule, when there is a conflict between the Zohar (which was authored by Rabbi Shimon) and the Talmud, we follow the Talmud over the Zohar. In the heavens, however, Rabbi Shimon’s rulings – both in the Talmud and in the Zohar – are accepted as authoritative. Therefore, in a sense, Rabbi Shimon’s death was a joyous occasion for him, as he entered the heavenly realm where his rulings were accepted, as opposed to the earthly realm, where his opinions were not followed. We therefore celebrate Lag Ba’omer as a happy occasion, marking Rabbi Shimon’s entry into the heavens where his rulings were deemed authoritative.
In light of the festive nature of Lag Ba’omer, Tahanunim (supplications) are omitted from the prayer service, and eulogies and fasting are forbidden.
Summary: The custom of the Sepharadim is to allow shaving and haircutting only on the 34th day of the Omer, and even if Lag Ba’omer falls on Sunday, shaving and haircutting are allowed only on Monday morning. It is customary to light bonfires and to visit Meron in Israel on Lag Ba’omer. More generally, this day should be observed as a day of prayer, Torah study and spiritual elevation. Tahanunim are omitted from the prayer service, and fasting and eulogies are forbidden.