OK, it should be a bit easier to install now. I believe it is 100% xhtml compliant as well.
https://www.ubbdev.com/mods/6/calendar.zipI included the template files pre-done and updated the instructions so that you are requiring the templates into the posting pages instead of adding the contents to the .php files. We are using these same files here.
Next up will be pulling the text out to make a language file... I'll work on it this weekend or so unless someone else wants to jump in there
Max feel free to do with this how you want, thank you for your work in getting it to v6 status