I don't know what place you are located right now or maybe in connection to places when saying about "fly day". I think this is what is all about, if you already heard about this word before. In my birth country, it is very common of folks joking of fly day as from the word "Friday" got from the kids' threat about aliens and such. On the other hand, the good thing is fly from the office work stress because it is already Friday, the last day of work to which then the day after it, will be Saturday, the first day of weekends!
And should I have to say, Happy Fly Day everyone! Oops, Happy Friday, I mean. *wink* Enjoy! Smile.