When creating a Snapshot schedule go to the GUI in the following path:

MAINTENANCE -> snapshot -> Snapshots -> [snapshot_name] -> Function: Create schedule for snapshot task

Select the day or days for the time value. If you select no end time then the Snapshot will overwrite the next scheduled time.

For example:

M-T-W-T-F

Start Time 9AM (no end time)

Next day at 9AM will overwrite the previous Snapshot.