Why celebrate Yom Yerushalayim

Why didn’t King David establish a “Jerusalem day” (Yom Yerushalayim) on the day Jerusalem was inaugurated for generations, as the Rabbis established these days? If the answer is that he was waiting for the construction of the Temple by King Solomon, why do we celebrate before the Temple is built? If the celebration today is because the rescue of the Jewish people, also on King David’s times the was a rescue and still, it wasn’t celebrated for generations.

The prophets determined to say Halel for every great salvation (Pesachim 117 A). We do not have the list of days they said Halel, but certainly they did celebrate. However, there is the scroll of Ta’anit which remained from the Second Temple. That scroll is a list of all the days of salvation in the times of the the Second Temple, which were celebrated, and fasting and mourning on those days was prohibited. The most known holiday, which remained to future generations, is Hanukkah. With the second temple destruction, the Ta’anit scroll was cancelled, except of Hanukah. To the heart of the matter : I do not understand the question. We were all in danger of annihilation and we were saved, won and released of parts of the Land of Israel and especially Jerusalem. Is there any doubt whether to thank God for all these? Rabbi Ya'akov Ariel