In browser navigate to Advent and open
'Advent Calendar'  gives the full experience of no 'cheating'/ opening a window early
'Advent calendar old' allows any window at any time so it can be used yearround.
on desktop   the days open in a new window so the window needs to be closed (no back arrow) the selection screen is still open
not sure the behavior on tablets and phones
There is autoplay background music on each page BUT browers today block autoplay you can change the defaults for a site 
(for me on firefox the easiest  is from wsperati.name tools->page info->permissions tab-> autoplay uncheck use default  select allow)

From the web: how to allow autoplay on Crome
   " Click the padlock in the address bar and then click 'Site Settings' which opens in a new tab.
    Look for the Sound option and change from 'Automatic (default)' to 'Allow'.
    Now close that tab, which should return you to the previous tab. Chrome will prompt you to reload that page, so click 'Reload'.

 Each page also has a little icon in the address bar that shows blocked auto play and you can turn it on for that page but every new page (every day) will need that done. or on the page you can just hit the play arrow but that isn't autoplay.

There are index files for the stories, and carols (mp3)and pictures. (XMAS INDEX ... .txt)
The long text files are .odf but should open in Word depending on your defaults

There is a page just for the Dicken's Christmas carol at the bottom of the selection page,  full Gift of the Magis text is from Gutenburg.org