There is a custom, which originates from the Arizal (Rabbi Yishak Luria of Safed, 1534-1572), to cut a boy’s hair for the first time when he reaches the age of three. It is recorded that the Arizal took his son when he reached the age of three to the burial site of Rabbi Shimon Bar Yohai on Mount Meron (in Northern Israel), together with his whole family, and gave him his first haircut on that occasion. The Arizal made a great celebration and feast in honor of the event. It thus became customary among many communities to give boys their first haircut on their third birthday, or on their third Lag Ba’omer at the tomb of Rabbi Shimon Bar Yohai.
One of the sources of this custom is the Misva of Orla, which the Torah introduces in Parashat Kedoshim (Vayikra 19:23). This Misva forbids partaking of fruit produced by a tree during its first three years after it is planted. Since a human being is compared to a tree, as the Torah says in the Book of Debarim (20:19), "Ki Ha’adam Etz Ha’sade" ("for a person is [like] a tree in the field"), his hair corresponds to the fruit in the branches toward the top of the tree. Therefore, a child’s hair is left untouched for the first three years, just as the fruit of a tree are prohibited during the tree’s first three years. Furthermore, the Torah says in the context of the Misva of Orla that on the fourth year, the fruit becomes "Kodesh Hilulim Le’Hashem" – "consecrated for praising God." Usually, it is when a child reaches the age of three, and begins his fourth year, that he is taught his first words of Torah and begins learning the Hebrew alphabet. He thus resembles "fruit" in that he begins praising God through his Torah study in the fourth year. And since Rabbi Shimon Bar Yohai dedicated his life to praising God through his intensive Torah learning, the child is brought specifically to Rabbi Shimon’s burial site, on his "Hilula" – which resembles the word "Hilulim" – for his first haircut.
Additionally, as a child’s training in Misvot normally begins at age three, he is given his first haircut at this age so he can be shown the Misva of "Pe’ot," the requirement for a man to leave the sideburns when his hair is cut. This Misva is part of a boy’s Misva training which generally begins at age three, and thus he has his first haircut at that age.
The custom of the Sepharadim is not to allow haircutting on Lag Ba’omer; we continue observing the mourning practices of the Omer until the morning after Lag Ba’omer. The question thus arises as to why we allow children to have their first haircut on Lag Ba’omer, when it is still forbidden for everyone else to have a haircut.
Hacham Ben Sion Abba Shaul (Israel, 1923-1998), in his Or Le’siyon (vol. 3, p. 184), explained that we may allow children to have their first haircut on Lag Ba’omer because the mourning practices during the Omer period to begin with are only customary, and not a strict Halachic requirement. The observances during the Omer period differ from the similar restrictions that apply in the beginning of the month of Ab, which appear already in the Mishna. The mourning practices during the Omer are not mentioned in Talmudic sources, and are rather observed on the level of Minhag (custom). And, moreover, there is a view, which the Ashkenazim follow, to allow haircutting already on Lag Ba’omer. Therefore, Hacham Ben Sion explained, there is certainly room to accommodate the tradition of giving boys their first haircut on Lag Ba’omer, even though others refrain from haircutting until the next day.
Summary: There is a custom which many observe to give a boy his first haircut on either his third birthday, or on his third Lag Ba’omer at the burial site of Rabbi Shimon Bar Yohai on Mount Meron. Sepharadim may observe this custom even though generally we forbid haircutting on Lag Ba’omer.
