Scotland, Northern Ireland and the Republic will play their Euro 2020 play-off semi-finals on 8 October.
The ties were initially scheduled for March, then moved forward to June.
Scotland host Israel at Hampden and, should they win, they would travel to face Norway or Serbia on 12 November with a place at the finals at stake.
Northern Ireland are away to Bosnia-Herzegovina in their semi-final, with the Republic travelling to play Slovakia.
The two sides will meet in a one-off final in Belfast the following month should they win their respective ties.
The matches will take place in the October and November international windows, with three games played rather than just the two 2021 Nations League contests previously scheduled.
The announcement means previously scheduled Nations League 2021 matches will need to be moved.
Uefa have delayed Euro 2020 by a year due to the coronavirus, with the tournament now taking place from 11 June to 11 July 2021.
The 12 original host cities have been confirmed as venues and the updated match schedule approved.
Were Scotland to qualify, they would be in a group with England, Czech Republic and Croatia, with the latter two matches at Hampden on 14 June and 22 June and the first at Wembley on 18 June.
And should either Northern Ireland or the Republic get through, they would join Spain, Poland and Sweden in a section played in Dublin and Bilbao.
News via BBC Sport