It comes every six months, but today is the day that another version of Windows 10 gets dropped from support. The versions that won't get serviced after today include Windows 10 version 1703 (Creators Update) and Windows 10 Mobile version 1607 (Anniversary Update).
The Windows 10 SKUs that are going away after today include Home, Pro, Pro for Workstations, and IoT Core. If you're on any of these versions, you should have been upgraded to a newer one by now. The Windows 10 Creators Update was a Semi-Annual Channel release, so there's no extended support.
If you're on Windows 10 Enterprise or Education, you don't have to worry. Microsoft announced last month that currently supported versions of those SKUs will continue to be supported for 30 months.
As far as Windows 10 Mobile version 1607 goes, this includes both Windows 10 Mobile and Windows 10 Mobile Enterprise. This is a notable end of life date, because there are a number of Windows phones that were never updated beyond the Windows 10 Mobile Anniversary Update.
Only 13 phones ever got the Windows 10 Mobile Creators Update. The only two that were upgraded from Windows Phone 8.1 were the Microsoft Lumia 640 and 640 XL; the rest shipped with Windows 10. That means that there's no way to get support anymore for premium devices like the Nokia Lumia 930, 1520, or the HTC One M8.
The next notable Windows 10 end of life date is for Windows 10 version 1709, which won't get support after April 9. Once again, this applies to Home, Pro, Pro for Workstations, and IoT Core.
For Windows 10 Mobile, version 1703 will end support on June 11. The only devices that will be affected by that are the Lumia 640 and 640 XL, as all other Windows 10 Mobile devices that got version 1703 also got version 1709. Windows 10 Mobile version 1709, which is the final version of the OS, will end support on December 10, 2019.
Presumably, Microsoft will release final cumulative updates for the versions that it's ending support for today, as it's Patch Tuesday.