Halachah and Tosfos in the Days of the Omer

Rav Menachem Mendel of Rymanov said that he never understood the secret of the matter until recently: the laws and halachos that were forgotten in the mourning for Moshe (see Temurah 16a) were restored to us by Osniel ben Kenaz during the days of Sefiras HaOmer. For this reason it is a minhag Yisroel, which has the status of Torah, to learn halachah and Tosfos during these days. This appears in a letter, Igeres HaKodesh, by Rav Yechezkel Paneth of Karlsberg, first printed in his Shu"t Mareh Yechezkel, siman 104. It is known that on the thirty-fourth day of the Omer, the yahrzeit of Rav Menachem Mendel of Rymanov, the Divrei Chaim of Sanz was found reviewing this teshuvah, and his eyes poured forth tears of longing.