You can detect the day of the week and redirect the user clicking on the link to a different URL dynamically -
onclick="var d = new Date();var n = d.getDay();
event.preventDefault() cancels the default action of the anchor tag.
This is kind of sneaky as the URL that the user sees in the status bar on hovering over the link will be different from the one he will be redirected to. But then, URL shortener services & Opera browser do such sneaky things too in a different way.
Don't know from which version of Opera this started but in version 16, the protocol and querystring part of a URL are trimmed.
Only if you select the URL will you be able to see the complete URL -