Microsoft 365 (Outlook Web App) - Import or Subscribe to a Calendar
In Outlook Web App, you can import events from an .ics file to your existing calendar or subscribe to a calendar online and receive automatic updates (known as an ICS subscription or an iCal subscription.) You can import calendars from websites or calendar programs that support the .ics file format, such as Apple iCal, Google Calendar, and Mozilla Lightning.
Visit the following Microsoft article for detailed steps and more information on importing or subscribing to a calendar.
Import or Subscribe to a Calendar