The Talmud in Taanis 29a says:
משנכנס אב ממעטין בשמחה
From when Av enters we reduce our happiness
משנכנס אדר מרבין בשמחה
From when Adar enters we increase our happiness
Since we are not told when we are to stop increasing our happiness, presumably we are to continually increase our happiness until Rosh Chodesh Av, when we are told to begin reducing.
So what do we do to celebrate the day before Rosh Chodesh Av, the happiest day of the year?