Green’s Dictionary of Slang

(orig. US) thank God it’s Friday.

1937		Panorama 27/2: ‘Thank God it's Friday Club’ [...] is fast spreading over the country. There are now something like 20 chapters. Friday afternoons are spent in drinking beer at some local dive.
1941		Star (Marion, OH) 13 Nov. 19/4: I thought I’d heard of everything in the way of boosterclubs, alumni organizations and the like, but this city, home of the OhioState university Buckeyes [...] has come up with one that tops them all. It’s the ‘Thank God It’s Friday’ club, composed entirely of undergraduates here at State. [...] A typical meeting of the TGIF club goes something like this [etc.].
1957		N.Y. Times 8 Sept. SM13: Newcomers to Patrick Air Force Base in Florida, where missiles are tested, are usually mystified by seeing the initials TGIF on bulletin boards and notices of various kinds. They are strictly non-regulation. They stand for ‘Thank God It’s Friday’ – meaning pay-day and week-end relaxation for some in the form of beach parties, club dances, and so on.
1967		Banchero & Flinn ‘Sociology and College Sl.’ in AS XLII:1 53: TGIF (Thank God it’s Friday). Used attributively to describe a party — usually a beer-drinking party — celebrating the arrival of the weekend: TGIF party. Sometimes verbal, as in ‘We went TGIFing yesterday.’.
2009		P. Temple Truth 280: This is the TGIF gang. Work people.