temporary banner or pop-up window for a temporary message

I have searched high and low in the forums, but since i am not sure what I am looking for is actually called, it's impossible!

I am having a big sale for 24 hours on my shop: http://shop.mortimersnodgrass.com
during the sale, I would like people to get to the site and have a banner, or actually, a pop-up window on top, with a quick message adressing the details of the sale, that they would have to click close to proceed.

I am certain this is easy, but I have no idea how to do it, or most importantly, what it is called, so I can look for the right steps.

thanks!

Technically possible... but you face other challenges:

1. Many people BLOCK popups.
2. Forcing a popup close before being allowed to proceed is one of the most annoying procedures and most people HATE it.

Rather just put a plain SALE notice on the home page, or create a sidebox with that info. Much less intrusive, and if in a sidebox, it displays all the time during a visit.