"Dolores Jane Umbridge (High Inquisitor) has replaced Albus Dumbledore as Head of Hogwarts School of Witchcraft and Wizardry."
-- Educational Decree 28 (OP28)
When the D.A. is caught at practice, Dumbledore takes the blame and leaves the school. By order of her own Educational Decree 28, Hogwarts High Inquisitor Dolores Umbridge becomes the Headmistress.
Unhappy with this development, the students soon revolt (OP28).
All we know is that, since the previous chapter took place on a Monday in April, this chapter must take place Tuesday and Wednesday of the same week. Hermione also says that the Easter holiday begins Friday, but since the calendar doesn't align to a real-world calendar, it still doesn't help determine exact dates.