Do you already have a way to get the date for next Friday? If not then take a look at this post and swap Friday for Monday.
Once you have the date for next Friday you can subtract the current date from it. You could do this in a hidden group of content type date and then use the group’s value in your text field.
You may also want to check out the Relative Tome plugin, which you may find useful for the second part above. Disclosure: it is my plugin, but it is free and I only mention it in case it helps.