A move by Delta Air Lines into new terminals at Los Angeles International Airport this spring will set off what LAX officials are calling “a move of airlines never before seen at a major U.S. airport.”

Some 21 airlines will be relocated to accommodate Delta’s move from Terminals 5 and 6 to Terminals 2 and 3, with much of the work taking place from May 12-16, officials announced Tuesday.

“Delta’s move is one more element of LAX’s massive modernization effort which continues the transformation of the airport and will improve the guest experience,” said Sean Burton, president of Los Angeles World Airport’s Board of Airport Commissioners. “While moving 21 airlines over three nights is a monumental task, Delta and the LAWA team have been planning and preparing for months to ensure a smooth transition.”

Delta is planning up to $1.9 billion in improvements over seven years to modernize and connect Terminals 2 and 3 to the Tom Bradley International Terminal. Delta’s flights will operate from as many as four terminals at times during the move.

The realignment actually began in January, when American Airlines exchanged four gates in Terminal 6 for four gates in Terminal 5 held by Delta.

Airport officials said it will be the largest relocation of airlines in the history of LAX, which is the second-busiest airport in the United States, and the fourth-busiest in the world.