I am rather surprised at the seriousness of this question. In England we live in a rural area and generally shop once a week. In Palomares we shop daily for bread but it is not the end of the world if we had to stock u for a couple of days.
The only reason I can see for shopping on Boxing Day is for the "January" sales in UK and you will not catch me ever doing that. I think in most places there will always be local shops or petrol stations open for emergencies,
I must admit I occasionally shop in a supermarket on Boxing day in case there are bargains in Meat or Fish!