Repeating entries in Calendar
The N900 Calendar app can create events with a limited range of recurrence types: every day, every workday, every week, every month or every year. However, the underlying calendar engine is capable of understanding more complex repetition types (for example every 4th week) which the Calendar app can display.
To get such an entry onto your device calendar, create it using Outlook (or other calendar app of your choice), export the entry as an iCalendar file and send it to your device by email. Tap the file in the email and select Open with Calendar, then select which calendar you want it to be in. Do no try to edit it on device unless you want the repetition to revert to one of the simple types available on device.