What's idiomatic of these two choices ("In what month" Vs. "In which month")?
I were told in the past that I have to choose "what" in a place where I don't know anything about the choices (for example: "On what floor are you?") and "which" in a place where I know about the a specific choices out of others that exist ("On which floor are you, first or fourth"?). Assuming that it is true, I am asking about months if it's the same, because I see not native English speakers that use both interchangeably especially about months.
"In what month do they celebrate this holiday?"
"In what month were you born?"
"In which month do they celebrate this holiday?"
"In which month were you born"?