British journalist James Cameron (1911-1985) was asked if he could come to New York City for an interview. He wrote in 1966:
 
“Naturally I jumped at it: I love short trips to New York; to me it is the finest three-day town on earth.”
 
A 2011 novel by Margaret Maron is based in New York City and titled Three-Day Town. Many websites change “town” to “city"and “earth” to “world”: “New York has been described as the best three-day city in the world, and that’s about right.”
 
New York (NY) Post entertainment columnist Leonard Lyons once wrote that San Francisco was a “three-day city, including the museums.” Beyond three days, Lyons would have nothing interesting to write about. In 1961, Lyons revisited San Francisco and said that he was wrong—it was a four-day city, in part because it added baseball’s Giants (moved from New York).
 
 
tripextras: New York City Travel Guide Information
New York has been described as the best three-day city in the world, and that’s about right. After a frenzy of museums, galleries, bars and clubs, many visitors are ready for a break. Fortunately there’s a lot of choice in excursions, from the National Parks of Upstate New York, to the beaches of Long Island or simply the leafy oasis of Central Park. Whatever you’re after, New York is ready and waiting to bewitch, bother and bewilder.
 
Wikipedia: James Cameron (journalist)
Mark James Walter Cameron (17 June 1911 – 26 January 1985) was a prominent British journalist, in whose memory the annual James Cameron Memorial Lecture is given.
 
Google News Archive
5 July 1961, Lawrence (KS) Daily Journal-World, pg. 4, col. 6:
The Lyons Den
Frisco Called a Three-Day City
And Columnist Evaluates Place
By LEONARD LYONS
SAN FRANCISCO: The opening page of Barnaby Conrad’s book “San Francisco” quotes me as describing it as “a three-day city, including the museums.” We’d been measuring various cities by the number of days a columnist could spend there before he exhausted the copy and he—or, what’s worse, his readers—became bored.
 
No matter what your curiousity about people and places, there nevertheless are major places which are three and two-day cities. And sometimes—because time is of the essence and the highlights are laid on—the mood and color of a place can be captured even in one day. We’re dealing in daily columns, not in learned treatises. Twice, I’ve flown around the world in 21 columns.
(...)
But, frankly, I am ready to concede error. San Francisco is NOT a three-day city. It’s a four-day city, because two things have (Col. 7—ed.) been added since my last visit—the Giants’ ball club and the North Beach colony of banjo parlors, poetry joints and unique havens where lonely, restless people can kill a night pleasantly without too long a haul.
   
Google Books
Witness:
Journey to North Viet Nam
By James Cameron
London: V. Gollancz
1966
Pg. 132:
Naturally I jumped at it: I love short trips to New York; to me it is the finest three-day town on earth.
 
13 July 1975, New York (NY) Times, “An Indian Summer” by Eden Ross Lipson, pg. 207:
Ten years ago, when he returned from Hanoi and reported that the American bombing had unified the North Vietnamese much as the Blitz had unified Britain, James Cameron, the English journalist, did a media tour of North America to explain himself. A sprightly television interviewer, strained by the painful subject of war, asked how it felt to be in New York. All right, he said, New York was all right “for a three-day town.”
   
29 November 2002, St. Albans (VT) Daily Messenger, “Sounds like a plan” by Leon Thompson, pg. 4, col. 2:
Journalist and author James Cameron once said he loved short trips to New York City, calling it “the finest three-day town on earth.” He’s right, of course. What journalist isn’t?
 
OCLC WorldCat record
Three-day town
Author: Margaret Maron
Publisher: New York : Grand Central Pub., 2011.
Edition/Format:  Book : Fiction : English : 1st ed
Summary: While in New York, Judge Deborah Knott has been asked to deliver a package to Lt. Sigrid Harald of the NYPD. Sigrid offers to swing by the apartment with her husband to pick up the box, but when they reach the apartment, they discover that the box is missing and the doorman has been murdered.