This seems like a specialized requirement, so I’d guess it’s unlikely there’s a plugin for it, but if you know a bit about CSS and PHP, modifying your theme for this requirement should be possible.
Off the top of my head, look in your theme’s header.php file for the line that pulls in the style.css file, something like
<link rel="stylesheet" href="<?php bloginfo('stylesheet_url'); ?>" type="text/css" media="screen,print" />
just below the <title>…</title> section. It may not look exactly like that, but somewhere in the header.php file there’s a link to your style.css file.
You can add lines there to pull in different CSS files depending on what your requirements are. For instance,
1. create a css directory in your theme directory (if it’s not already there)
2. in that directory create the css files
Sun.css, Mon,css, Tue.css, Wed.css, Thu.css, Fri.css, Sat.css
with the code you want to use on the different days of the week.
3. Then in your header.php file, use the PHP function date and add a line like
<link rel="stylesheet" type="text/css" media="all" href="<?php bloginfo('template_url'); ?>/css/<?php echo date('D'); ?>.css" />
Does that sound like what you want?
You could, of course, use a different function to select the auxiliary CSS file — and create those files using some other scheme.
