Q. Our son’s 3rd birthday Iy”H take place just before Pesach. I was thinking of making his upsherin at that time. 
My husband suggested we wait until Lag”Baomer which we be about four weeks after his birthday.
Is it any better to do it on Lag”Baomer ? If making an upsherin is a minhag why do some wait until Lag Baomer.
Is there any mitzvah of an upsherin. It’s our family minhig, so I want to keep it, but If the Rav has a moment can he further explain why families do it.Thank you!

A. On question 780 in regards to the source of this minhag we wrote: “It could be that this was done with the intention of being mechanech, educating and starting off the child on his very first mitzvos at a place that is a source for tefilos and is a Mokom Kodosh. One may add that the kever of Rashb”i was chosen in these generations, following what Rava points out in Talmud (Makos 17b) that women should strive to have children that are comparable to Rabbi Shimon Bar Yochai. It could also be that Rabbi Shimon gave an opening to return for people who are not yet Baalei Teshuva by upholding the opinion that “dovor sheino miskaven” (an unintentional act) is sometimes permitted. That would explain why this festive occasion became so popular even by the yet non-religious. The Chalaka would then be done on Lag Baomer the day his yorzait or the day he came out from the meara.(See Minchagei Tispores Rishono p.125, Hakoton Vehichosov p. 58, Nitey Gavriel – Pesach 3: ch. 58) Horav Shlomo Miller Shlit”a pointed out that traditionally some would do the chalaka at other k’vorim of other tzadikim such as the kever of Shmuel Hanovi. He also made reference to the Medresh (Tanchuma – Kedoshim 14 and Yakut Shimoni ibid.615) that compares a child to the mitzva of Orlo, where after the first three years you enter the fourth one designed as “Kodesh Hilulim.” That being the time of his beginning in chinuch for mitzvos he can now perform. (See similar opinion in Arugas Habosem O.H. 210).”
Poskim disagree whether it is permitted to do the chalaka in a Beis Haknesses or a Beis Medresh. Sdei Chemed (M. Beis Haknesses 10), Shulchan Gavoah (531) and others permit, while Lev Chaim (2: 172), Chino Dechaye (55), Meoros Nossan (112) and others prohibit. Nitei Gavriel (Tigalachas Hayeladim (8: 4) maintains that just cutting some of the hair, as is done to honor the important guest, is permitted by all.
Horav Shlomo Miller’s Shlit’a opinion is to avoid giving a haircut in shul, however in need, just cutting a small amount during the chalaka ceremony, would be permitted.
As far as waiting until Lag Baomer, the Rov maintains that one should follow his family traditions.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a