Calendar event collections are not available to use in collection displays.
While creating the Collection, and selecting Calendar event as item type, there is no further option as "Subtype" as with other elements like document or web content. The Collection Display Fragment on the content page does not display the Calendar Collection to select.

Liferay DXP 7.4

To be able to list the other missing collections when clicking “Select Collection” their type (assetEntryType) needs to be added to the "itemTypes" in the request's payload. (ex.: for the Calendar Event collection: "com.liferay.calendar.model.CalendarBooking")
As per the current implementation only the collections with assetEntryType AssetEntry, BlogsEntry, DLFileEntry and JournalArticle are listed among the selectable collection types. (There are other itemTypes in the request payload, but they can't be selected as collection type. ex.: "com.liferay.segments.model.SegmentsExperience", "com.liferay.portal.kernel.model.Layout")

While selecting 'multiple item types' and then 'calendar events' solves the problem, it is only a workaround.
When clicking on "Select collection" we list collections by their itemTypes, more specifically these types:

"itemTypes": [
    "com.liferay.journal.model.JournalArticle",
    "com.liferay.asset.kernel.model.AssetCategory",
    "com.liferay.portal.kernel.repository.model.FileEntry",
    "com.liferay.segments.model.SegmentsExperience",
    "com.liferay.blogs.model.BlogsEntry",
    "com.liferay.asset.kernel.model.AssetEntry",
    "com.liferay.portal.kernel.model.Layout",
    "com.liferay.document.library.kernel.model.DLFileEntry"
  ]

By default, the itemType of a Calendar Event is : "com.liferay.calendar.model.CalendarBooking". This type cannot be found in the above list, so it does not appear in the result.
When selecting  'multiple item types' and then 'calendar events', the itemType of the collection changes to  "com.liferay.asset.kernel.model.AssetCategory", which can be found in the list.
The real solution would be to include "com.liferay.calendar.model.CalendarBooking" as itemType (as well as the other types, that are missing from the list).
However, at the moment this is a missing feature.