Opener is another way of describing the first game of a tournament i.e. World Cup opener.
The 2022 World Cup has changed its opening date (opener) and will now open (start) on November 20th - a day earlier than planned).
- FIFA announced the schedule change on Aug. 11, moving Qatar's tournament opener a day earlier on Sunday, Nov. 20. (Sporting News.com, August 11th 2022)
Opening game is also used to describe this.
BTW, This is not a good look for the Qatar World Cup to change the starting date - what about fans travelling to the games? TV schedules?