4. A mourner may attend the synagogue services on Tish'ah B'av, in the
evening and during the day until the "kinos" are completed (1). He is
allowed to be called up to the Torah and to recite the Haftorah (2).
[Although normally this would be forbidden,] on this day we are all mourners.
(1) Normally, a mourner may not leave his house during the first three days
of mourning, even to go to the house of another mourner. There are
authorities who rule that this prohibition applies even on Tish'ah B'av
(and therefore, he may not attend synagogue at all). The Mishna Berura
559:24 cites Rav Shlomo Kruger's compromise ruling that on Tish'ah B'av, a
mourner during the first three days may only attend synagogue during the
day, when many kinos are recited (and he would be upset at missing the
opportunity to recite them with the congregation), but not at night.
(2) Permission for a mourner to be called to the Torah or recite the
Haftorah on Tish'ah B'av, is not mentioned explicitly in the Shulchan Aruch
or Mishna Berura.