Windows 10 availability date on 29 July 2015.
Users on Windows 7 and 8.1 have seen the Get Windows 10 App icon show up in their system trays over the last week. It is through this app that you can reserve your free copy of Windows 10 when it is available. This reservation is not necessary nor is Microsoft going to run out of Windows 10 upgrades either. One benefit of the pre-order is that the bits for the Windows 10 install will be downloaded ahead of the 29 July availability date. This means no delay in waiting for an approximately 3GB file to download to being the installation.