Q. We do not like like to eat “shum” or garlic to spice our salads, soups and meat with it. I Heard that there is a Minhag (tradition) to eat garlic on the night of Nittel. Is that really true and why?
A. Indeed, Poskim mention that some of the ones that keep the Nitel traditions. eat then garlic. Beis Yisroel (8: p. 301), explains that the reason of the tradition is to remind us how that night stinks for all of us, and how the Goyim who persecuted us made us feel like.
Nitei Gavriel (Chanuka p. 263 - 5: 5) adds that it is to avoid having marital relations on that night, for those that keep that tradition.
Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller, Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit'a