Little bit hacky but works a treat is to use metadata when Matrix is refusing to print simple HTML.
Place your HTML code as text metadata, and place this metadata into the prev/next field in the calendar asset. Matrix will print everything this way :)
I usually have 1 main metadata with the following fields:
1. Keywords - use for search engine
2. Description - use for search engines
3. Title - additional title / or for translation purposes
4. Image - twitter card / facebook open graph sharing
5. Language - language purposes e.g print en, de, fr etc into the body tag for styling and also into the metalang
6. Script - when you need inline script
7. Extra1 - for everything, but in your case you would put - <i class="icon-left-open"></i>
8. Extra2 - for everything, and <i class="icon-right-open"></i>