Brandon’s Buddies

if you are in the UK, your son will be offered a nursery space free of charge when he is 3, but it will be at an allocated nursery, probably one close to where you live, if you dont want him at that particular nursery say for example you want him in one next to your work, or his grandparents or at a siblings school then you would need to put his name down early for the nursery of your choice!
References :

It depends on the school when they take in children into their nursery class. Some schools take all the children from that year group all at once in the September after their 3rd birthday. So your son would be 3 and about 3 months, whereas some children will be almost 4, if they are a September birthday. Other nurseries take in new children once a term - once in September, once in January, and once after Easter. The children start the term after they have turned 3. So again, your son would start the Spetember after his 3rd birthday, and have one whole year in nursery. Wheras kids with a September birthday would start in January and have those 2 terms and then another whole year, because they are the oldest in the year. Other nurseries take in new children the half term after their 3rd birthday. So they have intakes in Septmber, after the Oct/Nov half term week, after Christmas, after the Feb half term week, after Easter, and after the May half term week. If your son’s birthday is before or during the May half term week then he may be able to start nursery after the half term week, and have half a term (about 7 weeks) and then the whole of the next year. However, I wouldn’t recommend starting him in the last half term of the school year as this is when the routine tends to go out of the window a bit as things like sports days and concerts happen, and practises are held for these, so for children beginning school at this time it is a bit bewildering. Plus, they are often only just starting to get used to school and then the summer holidays come and they have to start getting used to it all over again in September. But, anyway, yes, go to the school you want him to go to and ask to put his name down for their nursery class. They will inform you when he will be able to start. If he is able to start after the May half term but you want to wait until September then they should be able to hold the place for you - the school I taught at was very flexible with when parents wanted their 3 year olds to start as ours took in every half term, but some mums, very sensibly, thought their little ones weren’t ready at just 3 and requested to wait another term.
References :
Me - primary teacher, taught nursery last year