Tishah B'Av, which means the "Ninth of Av", refers to a traditional day of mourning the destruction of both ancient Temples in Jerusalem. Reform Judaism, however, has never assigned a central religious role to the ancient Temple. Therefore, mourning the destruction of the Temple in such an elaborate fashion did not seem meaningful. More recently, in Reform Judaism Tishah B'Av has been transformed into a day to remember many Jewish tragedies that have occurred throughout history.
Tisha B'av is July 31 - August 1 this year.
For more information on the Jewish Holidays, Reform Judaism.org.