The Next Run date determines when the database would be checked to see if it meets the purge requirements. If it does, the selected events are purged.
To have a purge job run sooner than a month, IE within 1-29 days, then Purge: Weekly should be selected.
On Day of Month should be selected based on the following criteria;
To run the job on a day within 1-7 days the 'On Day of Month' should be 1 Weeks
To run the job on a day within 8-14 days the 'On Day of Month' should be 2 Weeks thus setting the schedule for 1 week later.
To run the job on a day within 15-21 days the 'On Day of Month' should be 3 Weeks thus setting the schedule for 2 weeks later.
To run the job on a day within 22 - 29 days the 'On Day of Month' should be 4 weeks thus setting the scheduled date for 3 weeks later
On Days: would be the the days of the week that the job should run.
If you wish it to be ran monthly choose the day of the month you wish it to be ran on which is say the first of the month ex: Monthly | day 1 would run on the first day of the next month.