For those who started in September 2015, the deadline to register for P4 is 2016/04/08, thus your P3 should be done before that day.
P3s are organised by the students themselves, and the mentors need to attend. The chair person is not present. Thus, don’t wait for the central administration to book a room and give a time: this is your task to do so at time that suits your mentors and you.