Qt 5.6 hasn't even been released yet but with it running behind schedule, The Qt Company is looking to move Qt 5.7 along concurrently. As such, the Qt 5.7 Alpha source is now available while the official binaries are expected for release next week.
If you are a Qt developer wishing to test out the Qt 5.7 Alpha code, you can find the snapshots at download.qt.io. Assuming no major issues come up, the binaries for all major platforms of Qt 5.6 Alpha will be officially released next week.
Qt 5.6 is expected for release in the middle of March while Qt 5.7 is expected to be released around late May.