NewOutlook · Outlook Web · Outlook for Windows · Mail module Calendar Preview
Outlook: All Day events to show up more clearly in the Mail module Calendar Preview
All Day events and Out of Office time will display more prominently in the Mail module Calendar Preview, making it easier for users to identify scheduling conflicts when receiving meeting invites.
Microsoft's description
We're making it easier to tell when receiving meeting invites that conflict with scheduled Out of Office time or Busy All Day events, on Outlook Web and Outlook for Windows.