Thanks for the nicely done plugin.
After downloading, I noticed plugin was not functional for my installation.
I checked that code refers to the plugin files inside WP-CONTENT directory, which was renamed in my site for security reasons. I used content_url() function at all three places in merry-christmas-everyone.php where there was a WP-Content reference and plugin was usable normally.
Wished to share, may come useful for next version.