Rabbi Chaim Richman says that the holiday called Tisha B’Av should be changed from a day of lamentation and mourning to a time of anticipation to rebuild the Temple.

Temple Model from Israel Museum
When Roman general Titus Flavius Vespasianus destroyed the Temple along with the rest of Jerusalem, destroyed all the records and scattered Jews worldwide, the longing for a new temple began.
Rabbi Richman says that the holiday was not intended to be one of continued perpetual mourning. He maintains that it should instead motivate Jews worldwide to rebuild the Temple.
He continued that at least half of all Israelis want to rebuild the Temple.
It is our opinion that at least that many Bible-believing Christians want the same thing! Repeatedly when we speak to groups both in the US and Israel they want to talk about “when is that Temple going to be rebuilt?”
We heard it so often that we began doing a briefing just on that subject. It is serious stuff with lots of information.
So, according to the Rabbi you can mourn or you can pull for the people who want the new Temple built.