Woohoo! 
One way to host static HTML pages (or any file) is to put them in the static directory. For example, say you have a file on your computer called event.html, and you want to host it on your Micro.blog. Create a new template named static/event.html and paste the content there. The page will be reachable at https://your.micro.blog/event.html.
If you want a prettier and shorter URL without the .html file ending, you can name the template static/event/index.html instead. Then, the URL will end up https://your.micro.blog/event/.