it is not 24 hrs before departure. It is 1 day before. Suppose any train departure @ 9 PM from source on 03.06.2012 than tatkal booking will start @ 8 AM on 02.06.2012. Also if any train departure time is 4 AM from source on 03.06.2012, then also tatkal booking will start @ 8 AM on 02.06.2012. Hope it will clear the doubt.