Right around the time Spirit of the Century was published — maybe a little later, but if so not by much — Evil Hat stopped writing Fate as FATE. In essence, as Fate 3.0 came about (starting with Spirit of the Century), the old acronym the system had started with (first “Fudge Adventures in Tabletop Entertainment” and then “Fantastic Adventures in Tabletop Entertainment”), carrying up through Fate 2.x, stopped being a part of the name.
Old habits die hard, though. For the past 7 years many folks have continued to insist on writing Fate as FATE. It’s not “correct”, but it doesn’t really matter that that’s so. If folks want to write it that way for their own purposes, knock yourselves out. But if you want to portray it in a way that’s consistent with the official name as conveyed by Evil Hat: it’s Fate.
And now you know!