Microsoft flipped the switch on the Windows 10 upgrade right about midnight on July 28, 2015. I know this because my Windows 10 upgrade showed up at that time. I had just flown into NYC, sat through traffic due to a disabled vehicle in the Holland tunnel, and finally checked into the hotel when the upgrade showed up. The upgrade of my Surface Pro 3 took close to 2 hours, so I didn't get to sleep until about 2:30am. It doesn't take this long for everyone – it should only take an hour or so – but I was downloading over hotel Wi-Fi, which is notoriously slow no matter which hotel choose.
Even though the upgrade kept me awake so long, I had no issues with the download and install. Waiting so long for my upgrade to finish I had plenty of time to watch the social network wires and quickly found several upgraders with issues, and the common thread was error 80240020.
There's not much additional information given if you receive an 80240020 error, but it’s a notice that the Windows 10 download was either interrupted (unfinished) or corrupted.
To fix this:
Delete everything you find in C:\Windows\SoftwareDistribution\Download
Restart the download by heading directly into Windows Update
P.S. If the upgrade still doesn't work after following these steps, its likely you have an unknown issue with your network connection or hardware. In that case, use the new ISO tool to create an installation on actual media.