'Cause I don't like the days that don't start with "s"
Literally it's "days that end in i"... as in Romanian the days are:
Luni - Monday
Marți - Tuesday
Miercuri - Wednesday
Joi - Thursday
Vineri - Friday
Sâmbătă - Saturday
Duminică - Sunday
so it makes sense why he doesn't like the i-ending ones ;) btw in 4 of the days the i isn't actually pronounced but instead palatalizes (aka softens) the previous consonant, and in joi the i is pronounced as a y sound, like in boy