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If I’m a tiny bit slow and cranky, today, that’s because it’s the Jewish fast day called Tisha B’Av, which means the 9th of Av (today’s date on the Jewish calendar). To all of my Jewish friends, may you have an easy fast.
On this day, both the First and Second Temples in Jerusalem were destroyed. Five major tragedies happened to the Jewish people on this day, so we take the day to remember and mourn those and all the tragedies that have befallen the Jewish people. (On Tisha B’Av in 1492, the Jews were officially expelled from Spain, for example.) It actually is the end of three weeks of mourning and deprivation begun by another fast day.
We fast for almost 25 hours. The fast, as all Jewish holidays, started a little after sundown, last night and lasts until nightfall, tonight (about an hour after sundown). The fast day is usually in August, when the long summer days are the hottest and fasting is hardest.

Jewish fasts mean absolutely no eating, drinking, bathing (no showers), shaving, haircuts, laundry, washing, swimming, make-up wearing, sex, wearing of leather shoes, music, or entertainment for 25 hours. Since I eat very minimally during the summer, it’s not that hard. It’s the inability to drink water and the urge to eat some fruit as a snack and forgetting that I can’t that’s the difficult part. (We usually eat a bigger meal before the fast, which makes it harder because it expands your stomach, though that’s generally required for the Yom Kippur fast and not this one.) Also hard, since I’m a lip balm fanatic to remember not to constantly apply it, since I have tubes of the stuff in every room. It’s the breaking of habits and remembering to reflect that is part of the purpose of the day.
Here is an article from TIME Magazine from August 3, 1942, when Christian Ministers and Catholic priests participated in Tisha B’Av ceremonies for the very first time:

For the first time in history, Christian ministers participated last week in observing the solemn Jewish day of mourning, Tisha b’Av. Their action was expressive of the growing understanding among religious groups throughout the civilized world. Bishop Edwin V. O’Hara of Kansas City urged “all who profess the Christian faith [to] understand the declaration of Pope Pius XI that ‘all Christians are spiritually Semites.’ ”
Tisha b’Av, the ninth of the Hebrew month of Av (July 23 in 1942), is the saddest day in the Jewish year. Dating from the destruction of the First Temple at Jerusalem by Nebuchadnezzar, it also commemorates the destruction of the Second Temple by the Roman Emperor Titus, the expulsion of Jews from Spain in 1492.
Tisha b’Av is really the culmination of three weeks of mourning during which yeray shamayim (a pious Jew) does not marry, eat meat, bathe in streams, lakes, seas, cut his hair. On the eve of Tisha b’Av, he goes supperless to schul (synagogue), takes off his shoes, puts on his tefillin (phylacteries‚Äîleather arm bands used in prayer) and talis (prayer shawl). Then he squats on the floor and in the candlelit synagogue chants the Lamentations of Jeremiah (“How doth the city sit solitary, that was full of people! How is she become as a widow! she that was great among the nations, how is she become tributary!”).
Tisha b’Av is also the day when the Jews of Jerusalem gather at the Wailing Wall, all that remains of the Second Temple, to weep and pray for Zion. This year Tisha b’Av in Jerusalem was more solemn than ever, for there was scarcely a Jew who could not hear in imagination, above the ritual wails, the clank of Rommel’s tanks.

Tisha B’Av Observed Today at Israel’s Kotel HaMa’aravi (Western Wall)
More on Tisha B’Av at Judaism 101, My Jewish Learning, and Aish.