Conversation so far:
what year was the first temple of jerusalem destroyed
The Temple was destroyed in 586 BC by Nebuchadnezzar, the King of Babylon, when he conquered Jerusalem.
what is the day of mourning in relation to the destruction of jerusalem's temples
Every year, those destructions are marked by the day of mourning called Tisha B’av.

Question: how is the day observed
Self-contained rewrite: how is Tisha B’av observed