We write to everyone telling them how many hours they can have, which depends upon the age of the child.
Those who are in their final year before school are offered 15 hours. Children newly turned 3 are offered 10 hours.
We attach a plan of all the different sessions that are available and ask them to tick the ones they would like. There is also a place on the form for them to tell us about anything that would restrict the sessions that they could accept, e.g. work patterns.
Then as the forms are returned, I allocate the sessions. Generally speaking, this is done on a first come, first served basis, but I don't inform everyone of their session allocations until the closing date for the forms has passed. That way, if Child A has requested Mon / Tue / Wed, and then along comes Child B who also wants Mon / Tue / Wed but they've specified they can't do Thur / Fri, then sadly Child A will be bumped onto Thur / Fri.
That said, this is the first year since 2008 that I've actually had to do this.
If you're late returning your form, you get what's left.
If you're late applying, you get what's left.
I try my absolute best to accommodate people's requests (and usually succeed, plus I keep mini waiting lists for specific days, e.g. currently I have three children waiting for a Monday morning to come free). However, the financial sustainability of the preschool depends upon me filling all available sessions. We are always full.