Is your child at school, or pre-school (as you mention he's not there tomorrow)?
How old is he, and which year group is he in?
How do you usually find out about dates and events? Do you usually find out about things via this route in a timely manner.
The thing is, you have been given a parents evening meeting - you don't have to 'demand' one, one has been offered to you at 7.15 pm.
If that is not convenient for you, then you ask, politely, as a major favour, whether another time can possibly be arranged?
'Demanding' a time might be appropriate if everyone else had been given a time and you had not, or you had asked for one multiple times and it had been refused while all requests for others had been agreed.
It's not appropriate because the time that you have proactively been given isn't convenient for you. That's 'asking for a favour' time, not 'demanding' time.