Closing a key infraestructure for Christmas is not a tradition, it is a shame and to generate a problem to a lot of users, unbelievable coming from a country that is supposed to consider itself developed. Hello, XXI century?
It's uneconomical to keep the airport open as there would be very few flights, Ryanair stop flying around 3pm on Chrstmas Eve and don't start again until about 1pm on St. Stephens Day, Aer Lingus also don't operate as many flights on either the 24th or 26th, their flight schedule is significantly paired back due to lack of demand, the same with Stobart, CityJet and BA, people just don't want to travel in great numbers over these days to warrant putting on flights, they'd be loss making.
With that in mind, the DAA couldn't justify opening the Airport on the 25th just for the 5-6 airlines that might want to run flights (EK, EY, LH, DL, AA) if EI and FR et al all don't want to run their own flights, as they'd have to bring in a full set of workers to run the airport from security teams, baggage handlers, marshallers, ops teams, ATC, Fire Services, Fuellers, Checkin Staff, Engineers, de-icers, push back crews etc, and those 5 flights couldn't possibly cover the cost of bringing in all those staff, heating and lighting the terminals, lighting the airfield etc.
If they offered to those airlines to open the airport on the 25th on the condition that the airlines paid all the costs I'm sure the airlines would balk at the idea, as there's no way there'd even be enough demand to fill their own flights profitably, never mind footing the bill for opening the terminals and airfield.
You could bet your last euro if there was money to be made the airlines would be screaming out to have the airport open, but the numbers just don't add up. Aer Lingus and Ryanair have access to the figures and know how much it would cost to bring in people for the day. The comercial departments know even if there was enough demand to operate say 3-4 Heathrow flights, when you add in the cost of getting staff in to cover those three to four flights and turning the lights and heating on in the airport, it would be massively loss making.
It just doesn't make economic sense to open the airport, it's nothing to do with tradition, religion or unions, just pure cold cash.