The Weekend deal is normally announced via the Store front page and a news update pop-up when you start Steam. The deal is displayed on the top right of the store home page under the heading 'Spotlight'. It is normally announced between 11:00 and 15:00 PST (19:00 and 23:00 GMT) on the Friday and finishes sometime on the morning of the Monday after (normally after 09:00 PST/17:00 GMT).