You can set the period or frequency when the Periodic field is set to Yes.
Weekly
The forecast is repeated every N weeks. Check the boxes listing the days to select the day(s) of the event.
Monthly
The forecast is repeated every N months. The event occurs on the day of the month that corresponds to the accounting date or to the forecast date.
First
Select First and select a day; the forecast occurs on the first day of the month (for example, the first Monday of the month).
Last
Select Last and select a day; the forecast occurs on the last day of the month (for example, the last Friday of the month).
The value of N must be specified in the field Increment.