The most recent date I've seen is July 31, but that is NOT confirmed. I'd guess it's very unlikely it would be earlier than that.
If you buy a PC now with Win 7, 8, or 8.1, you will be able to get a "free upgrade" to Win 10, but no one yet knows the exact details on how that will work. It may or may not have a bunch of conditions and loopholes. It's all speculation beyond "free upgrade".
Nor does anyone know what the cash price for Win 10 will be if you want to just buy it later on.
Intel is coming out with a new processor line called "Skylake", probably in the July-September timeframe, so you might want to wait for that as well.