This indicates a fundamental misunderstanding of what Events ARE.
Events are pictures grouped by the time they were taken. Given that, of course you can't move photos from one event to another.
You can merge whole Events together by dragging one Event on top of another, but I suspect that's not what you want to do.
The actual solution here is to create an ALBUM. An album can have photos drawn from any event gathered together in any order or by any criteria you like, albums are entirely hand-chosen.
You also have the option of creating a smart album, where the photos that go in it are picked automatically based on a set of criteria you select. So, for example, "all photos with the tag 'birthday'" would be an example criteria of a smart album, and would automatically add any pictures with the tag "birthday" regardless of when they were taken.