I spent two Christmas Periods in the south of Spain. Christmas Eve - 6th January inclusive = Christmas in Spain.
Christmas Eve:- Celebrated and family members are welcomed home and is quite an occasion. Spanish places shut, Brit places open.
Christmas Day:- Celebrated but not to the extent that we do. Spanish places shut, Brit places open.
St-Stephen's Day:- Family visitation for Spaniards. Usually Spanish places open, Brit places open.
28th December (if it's 29th, Forgive me):- Diá de los Innocentes celebrated in thought of the innocent children put to death by order of Herod. It is a kind of Fools Day celebrated by children.
29th, 30th December:- Working Days, but Spanish places will close early (incl supermarkets).
31st December:- New Year's Eve, celebrated by ringing in New Year at midnight, local time. The Brits in Spain bring in the New Year in line with the BBC at 11.00 Spanish time. Spaniards celebrate with fireworks.
During all of the above the Spaniards might have their local Saints Day too. Everything Spanish shut, everything Brit open.
1st January:- New Year's Day. Spaniards probably will refuse to do business, the Brits will take anything on offer.
2nd January:- The Spaniards with difficulty haul themselves out of bed, leech to work wishing they were somewhere else. The Brits continue to take in any cash available.
3rd, 4th January:- Ordinary days. The Spaniards work, still do family visitation, but can't wait until the night of 5th January.
5th January:- Close anything that's open as early as possible except for the Brit places. This is the night of the Festival of the Kings. The Kings are paraded through nearly every town, village throwing sweets to onlooking kids. The kids in turn look forward to the morning of the 6th where gifts are exchanged.
6th January (Festival of the Kings):- Big Day for the Spaniards. This is what they've working towards. Massive feasts in every home. Gifts are exchanged. Their kids are like ours. Don't even think that any Spanish shop will be open. The Brits open as usual.
In general shopping centres are adorned with scenes of the Nativity and Flight Into Egypt. They have Christmas lights like us, but they make more effort. Because of the heat in southern Spain their flowers are still in full bloom. You would see sunshine most days over Christmas in Spain, but the nights are cold. I have used the Mediterranean for a Christmas Day and New Year swim.
I don't know if Grizzly can spend the whole month of December in Spain. He would probably pay the same rental for the whole month or the two chosen weeks. Flights get a bit dear from 14th December onwards so if he can spare the time, it would make sense to travel for 1st December and return to Ireland in or around 30th December. Rental for month of December can be had for about €550.00 + electricity charge if booked privately.