Update content of multiple pages/posts at once?

There are a few ways I might approach this.

The least intrusive would be to add a few empty spans or divs on your page and use the jQuery get method to fill these with text from the appropriate file.

Another possibility, but less secure would be to use a plugin that allows you to execute php code in your page. There are some serious security risks to this if you’ve got a group edited blog.

The most secure would be to define your own shortcode to do this, either as a custom plugin or in your theme’s functions.php.

All of these would allow you to bulk upload multiple days worth of files, and automatically load the correct one by day. The jQuery option would be the easiest to load the correct horoscope for the current visitor.