The WeekInterval property returns the number of weeks between instances of a recurrence pattern.
RecurringAppointment, RecurringEvent, and RecurringTask objects
Long
Read/write
When creating a weekly recurring appointment or task, set the WeekInterval property to the number of weeks that should occur between instances of the item. For example, to create an appointment that recurs every three weeks, set this property to 3.
This property is also used for monthly or yearly recurrences that specify instances in terms of weeks.