This is a yearly event, most stores and eateries have been 10-30% off all non-sale stock, and/or other deals/offers. Some give out free glasses of booze, cupcakes, pizza, crisps etc. Some have DJs on or other entertainment. Quite a buzzy atmosphere. Much of it, anyone can take advantage of like the free drinks.
The Arndale usually produces a leaflet around now, with information on all the deals.