Q. I have heard contradicting opinions as to when the mon began to fall. Was it on Pesach Sheni or was it Lag Baomer? Which is correct?
A. The Talmud (Shabbos 87b) writes that Bnei Yisroel arrived on the 15 of Iyar at Midbar Sinn, and that day was Shabbos. On that day Hashem promised them: “I am going to rain down for you bread from heaven, (Shemos 16: 4). Rashi (Shabbos 87b) explains that the monn began to fall next day, on Sunday the 16 of Iyar.
However, Chasam Sofer (Y.D. 233) quotes from Midrashim and Zohar, that after they ran out of bread, the next three days they did not have any. Only on the 18 of Iyar, which corresponds to Lag Baomer, the monn began to rain.
Ramban (Shemos 16: 12) explains that in the beginning they thought that the monn was only a temporary food. It was on Lag Baomer, that they were told that it will be their staple until they entered Eretz Yisroel.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a