Just to prevent you, it's the last day to upgrade for free to windows 10 from windows 7-8-8.1.
If you are undecided about that windows 10, you can always upgrade your actual OS to windows 10 just to activate the new license and after you can go back to your previous windows version. After upgrading to windows 10, you have 30 days to back to your previous OS.
This license is linked to your hardware, so if you decide to install windows 10 in 3 months per example (and you have always the same hardware), you will have already an effective license key for windows 10. Otherwise you will have to buy a licence for windows 10.