Since the sanctity of a holiday is derived not from the historic event it commemorates but from the revelation behind that event, some historic events do not deserve to be perpetuated as holy days at all.
A historic event may therefore be commemorated as a memorial day only, either sad or joyous as the case may be, but not part of the order of eternally sanctified days.
Thus the anniversaries of certain profoundly tragic events, like the destruction of the Temple, are counted as days of mourning throughout the generations.
Only when the world attains to a certain degree of redemption can these days be allowed to fade into oblivion.
Until then, certain days of the year, like the first part of the month Av, are considered days of mourning and misfortune, and in them calamities tend to appear, or reappear, so multiplying the force of grievous memories.
–Rabbi Adin Steinsaltz
From The Thirteen Petalled Rose by Rabbi Adin Steinsaltz