Upsherin
Upsherin, Opsherin or Upsherinish (Yiddish: אפשערן, lit. "shear off", Judaeo-Arabic: חלאקה, ḥalāqah) is a haircutting ceremony observed by a wide cross-section of Jews and is particularly popular in Haredi Jewish communities. It is typically held when a boy turns three years old. Among those who practice the upsherin, the male infant does not have his hair cut until this ceremony.
Background
The upsherin tradition is a relatively modern custom in Judaism and has only become a popular practice since the 17th century.
Yoram Bilu, a professor of anthropology and psychology at the Hebrew University of Jerusalem, suggests that there is little or no religious basis for the custom and its popularity is probably mainly social. The following are some quotes from his paper,
Rabbi Chaim Vital wrote in Sha'ar Ha-Kavanot that "Isaac Luria, cut his son's hair on Lag BaOmer, according to the well-known custom." However, the age of his son is not mentioned. An obvious problem raised by Avraham Yaari, in an article in Tarbiz 22 (1951), is that many sources cite that the Arizal held one should not cut one's hair for the entire sefirah – including Lag BaOmer, (see Shaarei Teshuva, O.C. 493, 8).