This well-known expression of relief was used for the first time by Jerry Healy (DJ on WAKR) - early 1970s. Then the movie with the same name - Thank God It’s Friday - increased the reputation of this expression in 1978. Donna Summer sang a popular song - Last Dance - on soundrack of this movie. A party song actually. After this year the phrase Thank God is Friday (TGIF) become very popular on college campuses in USA, being associated with Friday parties.
In the following years Thank God it's Friday was proliferated by sitcoms on NBC. The young people adopted it imediately. That is how I heard it for the first time :)
Have a nice week-end every one!!!