Originally planned for a November launch, the mysterious Zuma mission may finally go to space on Sunday evening. SpaceX has confirmed that its rocket, and the undisclosed national security payload, are ready for launch, and weather conditions appear to be generally favorable. The two-hour launch window opens at 8pm ET.

An undisclosed issue with the Falcon 9 rocket’s fairing caused SpaceX to delay the launch for several weeks in November and eventually move the date forward to January 4. Earlier this week additional propellant loading tests contributed to further delays, as did “extreme weather” at Cape Canaveral Air Force Station in Florida—mostly gusty winds.

But now conditions for the mysterious mission are 80-percent go, weather-wise, in Florida. This is SpaceX’s third classified mission, and arguably its most secretive flight for the US military. All that is publicly known about the Zuma payload is that it is a satellite manufactured for the US government by Northrop Grumman, and it is bound for low-Earth orbit.