Answer by taking a quote from the following article:

The Bee Gees were a pop music group formed in 1958. Their lineup consisted of brothers Barry, Robin, and Maurice Gibb. The trio were successful for most of their decades of recording music, but they had two distinct periods of exceptional success: as a popular music act in the late 1960s and early 1970s, and as prominent performers of the disco music era in the mid-to-late 1970s. The group sang recognisable three-part tight harmonies; Robin's clear vibrato lead vocals were a hallmark of their earlier hits, while Barry's R&B falsetto became their signature sound during the mid-to-late 1970s and 1980s.

In October 1999 the Isle of Man Post Office unveiled a set of 6 stamps honouring their native sons' music. The official launch took place at the London Palladium where the stage show of Saturday Night Fever was playing. A similar launch was held in New York shortly after to coincide with the show opening across the Atlantic. The songs depicted on the stamps are "Massachusetts", "Words", "I've Gotta Get A Message To You", "Night Fever", "Stayin' Alive" and "Immortality".  In 1978, following the success of Saturday Night Fever, and the single "Night Fever" in particular, Reubin Askew, the Governor of the US state of Florida, named the Bee Gees honorary citizens of the state, since they resided in Miami at the time. In 1979, the Bee Gees got their star on the Hollywood Walk of Fame. They were inducted in 1994 on the Songwriters Hall of Fame, as well as Florida's Artists Hall of Fame in 1995, ARIA Hall of Fame in 1997. Also in 1997, the Bee Gees were inducted into the Rock and Roll Hall of Fame. In 2001, they were inducted on the Vocal Group Hall of Fame. After Maurice's death, the Bee Gees inducted in Dance Music Hall of Fame in 2001, London's Walk of Fame in 2006 and Musically Speaking Hall Of Fame in 2008. And on 15 May 2007, the Bee Gees were named BMI Icons at the 55th annual BMI Pop Awards. Collectively, Barry, Maurice and Robin Gibb have earned 109 BMI Pop, Country and Latin Awards.  All three brothers (including Maurice, posthumously) were appointed Commanders in the Order of the British Empire in December 2001 with the ceremony taking place at Buckingham Palace on 27 May 2004. On 10 July 2009, the Isle of Man's capital bestowed the Freedom of the Borough of Douglas honour on Barry and Robin, as well as posthumously on Maurice. On 20 November 2009, the Douglas Borough Council released a limited edition commemorative DVD to mark their naming as Freemen of the Borough.  On 14 February 2013, Barry Gibb unveiled a statue of the Bee Gees, as well as unveiling "Bee Gees Way" (a walkway filled with photos of the Bee Gees), in honour of the Bee Gees in Redcliffe, Queensland, Australia.

When did they become honorary citizens of the state?
1978,