How to aggregate various Calendars from different data sources into a single SharePoint Calendar?

SharePoint Calendar is a great feature for teams and/or individuals who use SharePoint as a collaboration platform. It provides these teams a great tool to have one single place to check and mark the events which are important for the people involved in this group. It could be used for a recurring Team Meeting event, an announcement of a public holiday or even as a reminder to team members for a deadline.