When I enable a calendar I’m using for Holidays in Norway, the events show up as regular all day events. It seems eM Client does not have a special concept for integrating holiday calendars differently? 
In another calendar app I’ve been using, the holiday events show up in a special color, using a transparent/default background, so it’s less in your face than regular events. In this app I just choose which calendars to use for holidays calendars, it’s a property I can define per calendar IIRC.
Example of how it looks:
Setting per calendar:
If I’m right assuming no such functionality is implemented in eM Client, I’d like to add this to feature requests. 