Microsoft will be changing the look and feel of Outlook for Mac starting in mid-February. This new design is expected to become the default view by late February.
The new design includes improved search, improved meeting RSVPs and a "my day" view of the calendar.
More information is available on Microsoft's webpage about the new Outlook for Mac. Users can also preview the new design by clicking the "New Outlook" slider in the top right of their Outlook application.