Reading the manual, there is an option to run schedules in the last day, but I've tried and didn't work. Run = Level=Full Pool=Diaria on July lastday at 21:30
Scheduling the Last Day of a Month This version of Bacula now permits specifying the lastday keyword in the run directive of a Schedule resource. If lastday is specified, it will apply only to those months specified on the run directive. Note: by default all months are specified.