Most people I know (including myself) send their children to nursery at about 2.5. It doesn't have to be 5 days a week, you can do it just for 2 or 3 half days per week (depending on where you find availability).
I think it's really good for them to be outside of the comfort of their own home/nanny, to learn to interact with other children and be independent. In my experience, they spend the first trimesters learning basic thinks like sharing, taking turns, etc. And then at 3.5 they start learning letters, numbers, drawing, etc.
Having said that, I am sure that starting nursery at 3.5 can work well as well, if your child is already quite independent and you have a nanny who can do the educational stuff well.
Not sure about the demand for nursery at 2.5 comes from SAHMs
...I work full time but was quite keen on starting at 2.5 because at nursery they have a better chance of learning IMO (they are better behaved generally than they would be at home).