Hi @quasi
wp_list_categories unfortunately does not support any custom “query” parameters. So if you want to show only the categories of todays posts I think your only option is to loop through all of the active posts. Add their categories to an array and then loop through that array as sort of your own wp_list_categories function.
Taking it one step further it would probably be a good idea to save this array to a transient that will update each day. https://codex.wordpress.org/Transients_API
That will avoid any performance issues this would give you if there’s a lot of posts or categories.
