Are you involving your class in presenting this assembly? I just had a thought that, if you are, you could maybe do some sort of living advent calendar with them. If you could find or make something to represent an advent calendar door, some sort of movable screen, or stage curtain perhaps, you could have children showing different aspects concerned with advent each time the 'door' of your calendar was opened. You could make different large numbers to go on the 'door' to show progression towards Christmas as well, if you wanted. This is not something I've done, but, as I say, the thought suddenly occurred to me that it might be a feasible idea if you could have something big enough for children to hide behind which is easily movable. Good luck, anyway, with whatever you do!