When the Falcons host the Packers for the first regular season game ever at Mercedes-Benz Stadium on Sunday, it's going to come with a twist: The retractable roof at the new $1.5 billion facility won't be closed. 

Since officially opening for a Falcons preseason game on Aug. 26, the stadium has hosted a total of four football games -- two college games and two preseason games -- but the roof has been closed for each game because it's not fully functional yet. It currently takes hours to open or close the roof, which means the decision has to be made well in advance on whether it's going to be open or closed for a game. 

Once the roof is fully mechanized, it should be able to open and close in less than eight minutes. 

Here's what that will look like. 

Although the roof still isn't fully automated, the Falcons said that work "has progressed" enough for it to be open on Sunday as long as the weather cooperates. The current forecast in Atlanta is calling for clear skies in the evening, which means the roof should be open when Atlanta's game against Green Bay kicks off at 8:30 p.m. ET on NBC. 

If the roof is open for Sunday's game, it will mark the first time in nearly 26 years that the Falcons have hosted an open-air game in Atlanta. The last open-air NFL game played in Atlanta took place in December 1991.

The Falcons actually used to play all of their games outside before moving into the Georgia Dome in 1992. From the team's inaugural season in 1966 until 1991, the Falcons played at Fulton County Stadium, which you can see below. 

The Falcons moved out of that stadium in 1992, and then played in the Georgia Dome until January 2017. The team's final game at the Georgia Dome was a 44-21 win over the Packers in the NFC Championship. Fittingly, the Packers will also be the opponent when the Falcons play their first regular season game at their newest stadium. 

Although the roof will be open, not every problem at the stadium has been solved: Chick-Fil-A will be closed since the game's on a Sunday.