The DayInterval property returns the number of days between instances of a recurring item.
RecurringAppointment, RecurringEvent, and RecurringTask objects
Long
Read/write
The DayInterval property is valid only if the RecurringType property is set to Daily. When creating a daily recurring appointment or task, set this value to the number of days that should occur between instances of the item. For example, to create an appointment that recurs every three days, set this property to 3.