Millhouse: I don’t know about Mazals and such. I know that Yom Kippur is an Issur D’Oraisa and that Tishah B’av is because we act like we’re Aveilim. That’s entirely different from allowing a Minhag to make us be M’vatel a Chiyuv/Mitzvah D’oraisa. See Tosfos Bava Basra 2a V’havein.
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