Qué vs. Cuál

Question

Qué semana te vas de vacaciones?What week are you going on vacation?There are 52 weeks in a year.Apparently, 52 is a large enough universe to use Qué and not Cuál.If you wanted to say:  What day of the week is today?....would you say Qué dia de semana es hoy? or Cuál dia de semana es hoy?There are 7 days in a week.Is 7 a small enough number to use Cuál? or....Are you asking for a definition of the day and therefore Qué?

Accepted Answer

Hola, Andrew: What a good question!
In terms of the 52 weeks of the year, the choice of qué or cuál is the speaker's choice according to how he sees the situtation:
¿Qué semana es hoy?  = It is a general open question.
¿Cuál semana es hoy? = the speaker is thinking of the weeks as part of a group of options as  which week of the project is this one? or which week of summer is this? in the speaker's mind weeks are part of a definite group.
So we use qué with general options and cuál when the options are less (even if this is just in the speaker's mind).
Other rules that might help you:
To ask for definition with verb SER, we use qué: ¿Qué es un perro?
To ask for information with verb ES, we use cuál: ¿Cuál es tu nombre?

Answer

When 'qué' precedes a noun it asks about identity but not from a selection from a set like cuál does. Qué día es hoy? is the simplest way of asking that question.

Answer

Thank you for helping me!But.....I still don't understand. Both Qué and Cuál can precede a noun.Argument #1 in favor of Qué:  you are asking how you define THIS particular day.Argument #2 in favor of Cuál:  you are asking to select the one day out of 7 days in a week which is THIS particular day.Why is #1 more compelling than #2?Is it just "a rule" that anything having to do with time uses Qué?If so, is it possible to construct a sentence about time using Cuál?Wouldn't you say: Cuál dia es mejor para ti: lunes o martes?