Microsoft confirmed during its keynote at the Build conference last month that the next major update for Windows 10 is going to be called Windows 10 Anniversary Update. This name was chosen because the update is going to commemorate the launch of Windows 10 and Microsoft did say that it will release this update in the summer. Microsoft is now expected to release the Windows 10 Anniversary Update by late July.
Despite naming it Anniversary Update and saying that it will be released this summer, Microsoft hasn't provided a confirmed release date for said update, but insiders at the company cited in this report claim that the current road map is to finalize the update by mid-July.
However, timelines are fluid and prone to change, and road maps are meant for guidance only so nothing is set in stone just yet. If any unforeseen issues come up it's possible that the release might be delayed beyond July.
It merits mentioning here that Microsoft released Windows 10 on July 29th last year so many expect that given this is the Anniversary Update, Microsoft might be looking to release it on July 29th, 2016 to commemorate the one-year anniversary of Windows 10.
Microsoft obviously hasn't confirmed any of this but it all seems plausible.