Japanese and White Onions from sets..

I've never grown Onions before... well I have got Spring Onions in my veggie planter and they're doing well. I have just aquired some Japanese and White Onion sets... exact variety un known and I want to have a go at planting these. I will be planting these in containers so yes I know my yields will be relatively small but I use alot of Onions so it will be worth my while I think/hope I have greenhouse staging outside and I am planning to grow my Onions in troughs on the shelves.

The packets say I should plant the sets between September and December is this right? Whats the best time? and how long between planting and harvesting? Am I right in thinking I'll have Onions June/July next year? And can I plant closer together than reccomended and use the thinnings as salad onions?

Thanks