Below is a job that will run on 15th day of month. If the 15th falls on a none workday the LESS 0 WORKDAY statement tells the job to run on the previous workday, which will be Friday.
RUN 15TH DAY OF MONTH LESS 0 WORKDAY
If you want the job to run on Monday you would use PLUS 0 WORKDAY
RUN 15TH DAY OF MONTH PLUS 0 WORKDAY