I was wondering that too, as we were planning to watch Happily Ever After that evening so have had to change our days around... Given that this is an oddity, with park hours posting 8pm close or later on all the preceding and following nights, I guess that it is either a private event that night or that there might be an after hours event planned that has not yet been announced?
I like the idea of Lundi Gras though...! Haha!