Microsoft may be targeting a late 2015 launch for Windows 10, but the next Windows Server release will not happen until 2016.
Since the time of Windows Vista, Microsoft has been launching the Server variant of Windows alongside the normal version. However, this time the release of the next Windows Server OS won't coincide with that of Windows 10.
Microsoft announced on its Technet blog that they would be releasing previews of the next Server OS throughout 2015 and release the final bits in 2016. The preview of Windows 10, on the other hand is expected to end in April, with a release planned for late 2015.
The blog post has also revealed that the next version of System Center Configuration Manager for Windows 10 will be launched in the same timeframe as Windows, in order to enable the special update system that was announced earlier.
We haven't heard a lot about Windows Server since the availability of its technical preview back in October, but Microsoft has promised to reveal more about it in May at the Ignite Conference.