You don't necessarily buy card any more. you can go to a shop and buy minutes that are automatically sent to the phone if you have the number. If you don't have the number you can buy the minutes and get a receipt with a PIN that can be uploaded on the device.
There will be a number of shops open on Christmas day.
If you know their numbers you can purchase minutes online via the Hot Guide and if you want they can be posted to their account with a Christmas greeting from you - just add the greeting in the comments box.