Headfairy:
Although your LEA will publish the dates of the school year - it is down to each school to organise INSET (teacher training days - 5 a year), etc...
Each school is different regarding taking in Class R - some take everyone from day one - others start with some visits (half-day or full-day) and build up to the full week and full numbers.
So step 1) Inform your employer that your child will be starting school in September and you will be requiring at least 1 day (his/ her first day of school) off. However you can't commit to what day that will be because 1) you do not know which school he will be going to and 2) you do not know the schedule of entry for the new class yet.
It may also be the point to request 'flexible working'. If you pre-arrange with your employer that because there will be times when you will need to attend parent/ teacher meetings, assemblies, learning support clinics (parents come in and learn more about phonetics, for example, and then work with their child) - however, you will make up time off for such activities by coming in early, working late, etc... around this.
When a place is offered the school will contact you and will give you more information about how things will work. Usually there are some visits to the school and parent/ teacher meetings to the school in June/ July. They also will give you an idea of how things will start off (for example our school told us that the first week of school our children will only attend 1 1/2 days each to start to get used to the idea and then would go full time from week 2).
If the school haven't been clear about how all this is going to work - ring the school your child will be going to and explain that because your employer is requesting you confirm the day off now, you need to know the date of your child's first day of school.
If you haven't looked into it - as soon as you have your place - if you need before/ after school child care - contact the provider to ensure that this is all organised and in place before your child starts school. It will take a lot of the stress out of the situation.