Here’s a step-by-step guide on diagnosing and resolving the issue with your Microsoft calendar:

1. Check your mobile calendar settings:
   Check if the device calendar settings are set up to show all-day events from your Microsoft account correctly.
   
2. Update your device software:
   Update your device software to make sure that you have the latest bug fixes and security updates.
   
3. Check the applications you’re using:
   Make sure that the applications you use to access your calendar information are up-to-date.
   
4. Check your Microsoft security settings:
   Check your Microsoft security settings to make sure your device is authorized to display all-day events.
   
5. Check your device security settings:
   Check your device security settings to make sure they are set up to allow access to Microsoft services.