Unless your product is seasonal (productivity app for Christmas shopping?) - launch as soon as you have a viable minimum product (one that delivers value to the customer)
And Vanco is right - the Internet is global.
If it is ready, then launch it.
Assuming you are ready, the sooner you launch, the sooner you get feedback and you can start making the neccessary changes. Also, it takes a while to build up traffic, so you will gain some valuable insight into what people want and how best to market your website.
You may be feeling a little apprehensive about your adventure, but that is normal. Don't let the fear of failing stop you. Nothing ventured, nothing gained.
Go, go, go!
Launch as soon you ready, but hold your marketing efforts until fall when people can really pay attention to it.
We from our side, have decided to launch next month on a small scale but to postpone a press release until September. That gives us insight in user behaviour and allows us to finetune our offering, but we won't waste our sole opportinity for press coverage in the vacation period.