Women do not immerse in the mikveh on the night of Tisha B'Av and the mikvehs are closed.
Immersion should be when the fast ends, after you break your fast. Since you will already have finished counting the
seven clean days on the eve of Tisha B'Av, you do not have to wear white underwear or do additional
bedikot on the day of Tisha B'Av.
See
here for more information about preparing for mikveh when you immerse after the fast.