This question seems to be in conflict with your other one.
Your other question implies that you're not contracted to work on Wednesdays. If that's the case, your boss can't insist that you work on 26 December this year (unless there's specific provision in your contract to enable him to do so). However, if Wednesday is a normal working day, your employer can insist that you go in and there's no obligation upon him/her to pay you at a higher rate than normal.
Chris