Dear Rabbi, I asked myself for a long time concerning festivals and days of fasting that were instituted after the giving of the torah and remembrance of events that took place chronologically after the Chumash, for example Channuka, Purim, Taanit Ester, 10 Be’Tevet, Tisha Be’Av. From what I understood, those days were instituted by the sages(?) and I also heard that we are to remember a rescue of the Jewish nation (which would include Purim and Channuka), but I never understood which commandments and verses of the Torah (!) are the basis for instituting these festivals and days of mourning (and repentance). Thank you very much in advance!
The Oral Tradition teaches us to follow the “direction of the arrow” found in the Torah which tells of different holidays established for different occasions. For example, the Torah declares that every year on Pesach (Passover) we celebrate our freedom from bondage so the Talmud teaches us, how much more so we are obligated to annually celebrate Purim (and Chanuka) where we were saved not from bondage, but from actual death (M’gilla 14a, the Chatam Sofer says this kal vachomer is even m’d’oraita, Resp. Y.D. 233). The idea of thanking G-d through different blessings for food and the Land of Israel (e.g. Dvarim 8, 10; and ch. 26), and even for finding the proper bride (Breishit 24, 27) is obligatory and the minimum of common decency (to say “thank you”, to man and G-d). How much more so should we give thanks for salvations greater than simple food! The Torah teaches us not only the legitimacy but the value of spontaneous celebration and thanks for salvation (e.g. Shmot 15, 1; Bamidbar 21, 17), and it was the prophets (not later-day sages!) who officially established this practice by instituting singing the Hallel (King David’s songs of thanksgiving upon his victories) on any future national salvation (Psachim 117a). Accordingly, the poskim teach that throughout the generations there were national (Megilat Ta’anit) and local holidays instituted (Mishna Brura 686, 8) for salvations. Conversely, the Tanach (Bible) itself informs us that it was the prophets (not the later-day sages) who instituted Tish’a B’Av and the other annual fast days (probably based upon the idea of fasting for tshuva, as found in the Torah regarding Yom Kippur), and in the future, Israel has the permission and obligation to eventually “update” and turn those days into new additional holidays (Zecharia 8, 19). In short, the living Torah is not meant only to be studied but to emulate and “continue the arrow” through the decisions of the prophets and later, the sages.