Hola Delia
The answer is in this lesson that we are tagging on that line of the text:
Aunque + present indicative/present subjunctive
The speaker in this text is assuming that that bit of information (that they do eat it in other ocassions apart from December) is something known by the listener (in this case by the readers of this text), considering this as something known, familiar, background information, nothing new. This intention triggers the present subjunctive.
I hope it clarifies it.
Saludos cordiales
Inma
