Originally Posted by javabytes

The website upgrade text does not actually align with the upgrade windows. The upgrade windows still open at 5PM Eastern Time 6/4/2 days before the flight. The website language does not change from Upgrade Eligible to Upgrade Available until some time the next day, closer to the published windows. Regardless of the language displayed, the window is indeed open.

I can see a reason for this - you wouldn't want to start showing Upgrade Available until all the upgrades have been processed at the window opening. The process takes time - often 15-30 minutes but sometimes hours. You wouldn't want to show Upgrade Available, have a PM buy a ticket on that belief, only to have all the available upgrades go to DMs when the computer gets around to processing them 20 minutes later.