Earlier this year, Microsoft said Windows 10 would be out by the end of the US summer, which made it likely we'd see it before September. The latest evidence suggests it could land as soon as July.
Tech blog TweakTown notes that during its recent quarterly earnings call, processor manufacturer AMD said that it expected the "Windows 10 launch at the end of July". That would fit Microsoft's announced timescale, though it's at the early end of the date range.
We won't have to wait too long for confirmation. Microsoft's Build conference for developers, which takes place in San Francisco next week, is likely to see further announcements around Windows 10 release plans.