Bank Holidays are where life gets spicy. M&S Newtownabbey usually opens with reduced hours. Think 9:00 AM to 6:00 PM on a typical bank holiday Monday. But check the website first! No, seriously. I once drove there on Easter Monday, and it was like a ghost town. A very quiet, very empty ghost town with no chocolate eggs. Heartbreaking.
Christmas Eve? Expect to be out by 5:00 PM (or earlier). Boxing Day? They might be closed, or they might be open for the sales madness. Your best bet is to whisper a prayer to the retail gods and check the store’s official page. Or just call them. You can do that.