To prepare for this impending evacuation, the American embassy distributed a 15-page booklet called Standard Instruction and Advice to Civilians in an Emergency. The booklet included a map of Saigon which indicated rally points for helicopter pick up. There was an insert page which read: "Note evacuational signal. Do not disclose to other personnel. When the evacuation is ordered, the code will be read out on Armed Forces Radio (AFVN). The code is: The temperature in Saigon is 105 degrees and rising. This will be followed by the playing of I'm Dreaming of a White Christmas." The version they selected was sung by Bing Crosby. After the signal was given buses began to bring evacuees to the pick up points. It was called Operation Frequent Wind.
Radio was everywhere. There were radios in the PX, radios in the barracks radios in the offices for the REMF. It was the fastest and most certain way to convey a message to everyone simultaneously. On April 29th, 1975 At 10:51 AM Henry Kissenger gave the order and the signal was broadcast on AFVN. It's important to note that between 1973 and 1975, the station was operated by civilians at Federal Electric, a subsidiary of ITT. (Thank you N4UF)
Interestingly, Bing Crosby was privately opposed to the Vietnam War. Whether the administrator who made the pick knew that I don't know but I love the irony. In 2002, the Library of Congress selected the 1942 version to add to the National Recording Registry.
************************UPDATE***************************I was contacted by N4UF, an AFVN staffer with a slightly different version of events. But This version, despite contradicting popular sources comes from the most authoritative source possible: The last DJ: Chuck Neil. I'm quoting:
"We had a big Gates Automatic Programmer. We programmed most of our day on that machine. And I went back in there and took the cartridge with "105 degrees and rising" and "I'm Dreaming of a White Christmas," and popped it in the slot and punched it up. And that was my final act at the radio station...."Basically this means that Frank Snepp's account in "Decent Interval"was incorrect. AFVN did not air the Bing Crosby version. In an article by Chuck himself in the Library of Congress he states that he couldn't find Bing Crosby's version in the record library and substituted the Tennessee Ernie Ford version. Every other writer who used Snepp as a source passed on the error into history until in 2002 the National Recording Registry archived the wrong version of the song.