17th of Tammuz: Who Fasts and who is Exempt

 https://en.yhb.org.il/17th-of-tammuz-who-fasts-and-who-is-exempt/

In the intermediate situation – when the Temple is destroyed, but we are not plagued with harsh decrees, as was the case during Rabbi Yehudah HaNasi’s lifetime – the status of these fasts depends on the will of the Jewish people: “If they want to fast, they do so; if they do not want to fast, they do not fast.”  This is the law regarding the tenth of Tevet, the seventeenth of Tammuz, and the Fast of Gedaliah.  Regarding Tish’a B’Av, however, the matter does not depend on the nation’s will, and everyone is obligated to fast, even in the intermediate situation, because both Temples were destroyed on that day (Rosh HaShanah 18b).

In practice, the Jewish people are accustomed to observing all the fasts, even in the intermediate situation.  Therefore all Jews are obligated to fast on these days.  This is the halakha until the Beit HaMikdash is rebuilt, speedily in our days, when the fast days will become joyous festivals.