GLaDOS absolutely loathed Fridays. Why, you may ask, well it was because it was his favorite day. Who is he? He is that blithering idiot, Wheatley, who refused to leave the labs no matter what she tried to bribe him with.
Now, you may ask why Friday of all the mundane, meaningless days was his favorite. Was it because it led into the weekend? No, that couldn't be it, they do the same exhilarating thing everyday, testing the humans.
Well, it just so happens that the reason he adored that particular day of the week was because he had found a video file in the databases recently, a certain video file containing the most repulsive sound imaginable.
Yes, he had sumbled upon the music video the Rebecca Black's, Friday. He played it all day every Friday. GLaDOS was irritated to say the least, because of course he insisted on singing along to it, and he just had to follow her around all day to her absolute abhorrence. Yes, she truly despised Fridays and the torture they brought upon her.