The Rice University Neologisms Database

A clipping of the expression TGIF (Thank God It’s Friday); often used in place of Friday TG is a clipping of the acronym TGIF, meaning Thank God It’s Friday. Generally, it is used in place of Friday, even though the f, which stands for Friday has been deleted from the expression. Specifically, TG refers to good Fridays in which a lot of events are occurring. TGIF is now a hackneyed expression, whereas TG is more refreshing. Furthermore, TG is a lot funnier than TGIF, and thus sounds much happier, because it seems somewhat idiotic to shorten an acronym, and especially to remove the essential part of the acronym (i.e., the F that stands for Friday).