The fifth years take the practical portion of their Astronomy O.W.L. On the grounds below, Umbridge and others go to sack Hagrid, who fights them off and runs out the front gates carrying Fang. McGonagall tries to intervene on his behalf, is Stunned and taken to St. Mungo’s (OP31).
This chapter starts from the day after Gryffindor's Quidditch victory (Sunday, May 31), then goes through probably a week of studying for O.W.L.s and then almost two weeks of exams. We do know that Harry's last exam day, on which the next few chapters take place, is Thursday of his second week of exams - probably June 18. If the Astronomy practical is two nights early, we can suppose it might have taken place around June 16.