Taxis will be hard to come by on Christmas Eve. When we are in town, we typically host a pot luck with white elephant and invite people over, keeping in mind that the hardest time to find taxis is between 9pm and 2am. Buses and subte also stop running, but check those times on the city website. I’d recommend doing something similar over attempting to go out, especially if you don’t live near restaurants that will be open.
The morning of Christmas Eve, you’ll find confiterias, carnicerias, panaderias, rotiserias, and other specialty food shops open for limited hours as many will be buying their food for the family evening. The big chain grocery stores will also be open. The evening is typically a nice spread of Argentine appetizers, salads, entrees, desserts, and candy (turron seems a popular one); wines/drinks, exchange of gifts, fireworks (or watching them if not participating in the crazy off-the-terrace fun)...and spans several hours.It’s actually quite similar to my family’s festivities in the US, minus fireworks and swap food choices.
If you don’t cook, many of the specialty shops have good choices for finger foods and prepared foods (A’Manger is a good choice for olives, specialty meets, cheeses, and desserts, and they have a menu of items you can buy prepared to reheat later...everything is good there).
Load up on drinks and delicious foods Christmas Eve, and walk the silent streets Christmas morning for a coffee shop or confiteria that might be open. It’s quite amazing to see the contrast on Christmas Day and appreciate how busy this city usually is almost everyday of the year (minus paros).