We're expecting Windows 10 to be released this year, but Microsoft is yet to specify a precise launch date. It does look certain that it will be out by the end of August, however.
At the WinHEC hardware conference in Shenzen this week, Microsoft said that Windows 10 would be out by "this summer". In a northern hemisphere context, that suggests it will be out no later than the end of August.
That would give hardware vendors plenty of time to bundle Windows 10 on new computers for the Christmas market. It would also mean new phones running Windows 10 would beat the likely release of the next generation of iPhones in the final quarter of the year.