Hello Graham,
I hope all is well. I assume you are using (Outlook Calendar).
- First Open (Outlook App/Web): https://outlook.live.com/calendar/0/view/month
- Click on (new Event)
- Make Recurring
- Change settings to (Days) > and choose 28 from the dropdown list.
Tip: you can use your keyboard arrows (Up/Down) keys to change number.
Note: Review (Start Date & End Date) of that event.
- Save >> Save Event (Note: you can modify it later on as you like using same steps)
(Right Click > Edit)
If you are using (Mail App), I assume steps will be the same with different (GUI/Interface).
Let me know if you need any further explanation.
GoodLuck!
BR,
M