The origin of the term “Thank God it’s Friday” is unclear, but the earliest reported usage of the term appears to date back to 1941. The term first appeared in a newspaper article in the Marion Star of Ohio.
The author of the article expresses amusement at the existence of a Thank God it's Friday club on the Campus of Ohio State University.
Another source attributes the phrase to the Royal Canadian Air Force who claim to be the first to use the phrase. The Canadian military had a ritual Friday “beer call,” and the enlisted men coined the term to express relief that the work week was over and that their weekly ritual of drinking was at hand.