Rosh Hashanah 19B

וחכמים מטהרין.

whereas the rabbis declare it pure. According to them, it is still considered a broken utensil. R' Meir himself lived after the destruction of the Second Temple. The festive day commemorating the annulling of the decree of Rome was instituted as a result of an incident involving his student, R' Yehuda ben Shammua. From this, it is clear that Megillat Ta’anit had not yet been nullified.

תנאי היא. דתניא: הימים האלו הכתובין במגילת תענית, בין בזמן שבית המקדש קיים בין בזמן שאין בית המקדש קיים ⁠— אסורין, דברי רבי מאיר. רבי יוסי אומר: בזמן שבית המקדש קיים ⁠— אסורין, מפני ששמחה היא להם. אין בית המקדש קיים ⁠— מותרין, מפני שאבל הוא להם.

The Talmud answers: The question whether or not Megillat Ta’anit has been nullified is the subject of a dispute between tanna’im, as it is taught in a baraita: These days, which are written in Megillat Ta’anit, both when the Temple is standing and when the Temple is not standing, are days on which fasting is prohibited; this is the statement of R' Meir. R' Yosei says: When the Temple is standing, these days are prohibited for fasting because these days are a source of joy for Israel. But when the Temple is not standing, these days are permitted for fasting because these days are a source of mourning for them.

והלכתא בטלו, והלכתא לא בטלו. קשיא הלכתא אהלכתא! לא קשיא: כאן בחנוכה ופורים, כאן בשאר יומי.

The Talmud concludes: And the halakha is that these days were nullified, and the halakha is that they were not nullified. The Talmud asks: This is difficult, as one halakha contradicts the other halakha. The Talmud answers: It is not difficult. Here, it is referring to Hanukkah and Purim. These Festival days were never nullified, and Hanukkah is listed among the Festivals of Megillat Ta’anit. There, the halakha is referring to the rest of the days listed in Megillat Ta’anit, all of which were nullified.