Hello Brendan,
You need to specify a document type in your repeater to make it work. In you case probably following one:
ClassNames="CMS.Event"
However, I suggest you to store your transformation directly in the
Site Manager -> Development ->Document types -> <given document type> -> Transformation section and then link them in your repeater like this:
TransformationName="CMS.Event.default"
And the transformation code could look like this:
<div class="my_event"><%# Eval("EventName") %></div>
Best regards,
Jan Hermann