Hi Klaus,
Thanks for checking out The Events Calendar! That’s a very good question. There often are not hard rules about SEO. Generally speaking though, heading tags are used to present structure on the page to users and are essentially a document outline. Also since heading tags typically make text larger, this is also a visual cue to users and helps them understand something about the type of content underneath the heading text. With that being said, the theme you’re using should be applying heading tags to your site and The Events Calendar follows suit in that hierarchy of content by using H2 and lower.
Tuts+ has a great resource about when to use heading tags.
If you’d like to customize The Event Calendar template files, you can certainly do that. You can have a look at how to accomplish that over on our Themers Guide.
I hope that helps.
Kind Regards,
Jami