Calendar is an application which is integrated within Outlook, whether as part of the desktop Outlook App or on Outlook Web Access. It enables you to keep track of appointments, invite other users to meetings and organise your work schedule efficiently.
To use it, start Outlook (either the desktop app or Outlook Web Access). Click on the Calendar icon on the left of the screen. Dates can be viewed in Daily, Weekly or Monthly arrangements. Double-clicking on a time/date brings up a form to fill in details of an appointment.
It also has close links with Teams, whereby using the same account you can view and manipulate the same entries in Calendar. You can also use this view to set up a new Teams meeting, which appointment will cascade through to Calendar within Outlook