I have a lot of calendars. I would like to create a new one which aggregate contents from all of them.
Do you know how?
You need to create one "parent" calendar to which you overlay the other calendars.
"The following procedure describes how to overlay one SharePoint calendar onto another SharePoint calendar. After you overlay the calendar, you can use this calendar to create a special calendar view.
To overlay a SharePoint calendar with another SharePoint calendar, follow these steps:
In the Calendar Overlay Settings page, choose New Calendar.
In the Name and Type section, type the Calendar Name and then select SharePoint.
In the Calendar Overlay Settings section, type a Description for the calendar (optional), and change the Color theme for the calendar (optional).
TIP Selecting different colors for different calendars makes it easier to distinguish which calendar an event is from.
Type the Web URL for the SharePoint site that the calendar is in, such as https://contoso.com/TWGroupWorkSite/calendar, and then select Resolve. If the calendar is in the same site as the group calendar, Web URL will already be populated accurately.
Select the List drop-down arrow and then select the calendar that you want to add to the calendar.
Select the view that you want to use from the List View options.
If you want the overlaid SharePoint calendar to always display in the SharePoint group calendar, select Always Show.
Click OK to accept the overlay specification, and then click OK to complete the process."
Remember, the max calendars limits in SharePoint overlay is hard coded to be 10 and cannot be modified, if there are over 10 calendars, additional group calendar would be suggested, the following thread shows the similar issue, please refer to it for more information: