From this ‘Yenny’ cartoon:
http://www.comicssherpa.com/site/feature?uc_comic=csaeb&uc_full_date=20041003
Bablefish was no help. From the context, I’m guessing it’s a practical joke or surprise.
Parrandear is to carouse or to engage in revelry.
I would guess that parranda would be a carousal or revelry.
A Christmas "parranda’’, in this context, is a roving surprise party, popular in Puerto Rico and other Latino cultures, wherein a bunch of our so-called friends will show up unannounced over the course of the night at one or more other friends’ homes inflicting a Xmas party upon them – usually they bring the music and some of the drinks, you (the compelled host) have to provide the food and the rest of the drinks.
The joke here is based on that in Puerto Rico the parranda, back in more innocent times, was also given the name an “ASALTO”, literally, a stick-up or mugging – because, of course, your “friends” were showing up unannounced in the middle of the night to demand food, drink, and company – and would be announced with the word “asalto”. So, Yenny confuses her mother and sidekick’s cry of help with the announcement of a parranda, and while believing she’s chasing off some of her freeloader neighbors, she nails a real robber.
Just a bit like the chivaree of the mid west. Friends of the groom show up and announce themselves when the lights go out and kidnap the groom.
Real spoilsports. :rolleyes:
In some communities it the night before the wedding.
I’ll confirm that I asked my native-Mexican wife this, and didn’t get it at all. Gotta be some island (or as said, Puelto Riceno) usage.
In her Spanish, it’s mostly as tomndebb said. “Andar de parranda” is what you do all night when you’re just barely legal drinking age 
(or older, or on vacation, I’d admit).
Makes sense, thanks!