Question: Does a contemporary bet din have the authority to annul Tisha B’Av considering that the fast day is rabbinical, not biblical, in origin?
Answer: To answer this question, let us look at another. The Al Hanissim for Chanukah describes Bnei Yisrael’s actions in wake of the Chanukah miracle: “v’achar kach ba’you vanecha… – and afterwards, Your children came…” The Al Hanissim for Purim, however, contains nothing about Bnei Yisrael’s actions in wake of the Purim miracle. Why not?
HaGaon HaRav Hutner, zt”l (in Pachad Yitzchok, V’zot Chanukah, Ma’amar 14), asks this question and answers based on the general rule that no bet din may annul the decision of a previous bet din unless it is greater in wisdom and numbers (“b’chachmah and minyan”). If a navi sat on the earlier bet din, no later bet din can ever overrule it since the first one was clearly greater in chochmah. For this reason, all the decisions of the Anshei Knesset Hagedolah remain in force forever since nevi’im sat on the Anshei Knesset Hagedolah.
Thus, Purim, which was established by a bet din that included prophets, may never be annulled. Chanukah, however, is a different story. The Anshei Knesset Hagedolah no longer existed at that time as the age of prophesy had passed. Accordingly, a subsequent bet din could, in theory – at least at the outset – have annulled Chanukah.
For this reason the Al Hanissim for Chanukah notes that “ba’u vanecha – Your children came.” This phrase indicates that Klal Yisrael accepted Chanukah upon themselves. It became a widely accepted practice, and once this took place, no bet din could annul it. A minhag accepted by Klal Yisrael becomes an obligation.
For the very same reason – returning to our original question – no modern-day bet din may annul Tisha B’Av. Klal Yisrael has accepted Tisha B’Av upon themselves, giving it the force of an obligation.