On this day in 1517, Martin Luther posted his 95 Theses which he nailed to the Castle Church door in Wittenberg, Germany.
The reason for this 'nailing' is indeed a rather logical one, ie., when people had something of substance to say, they posted their ideas on the church door. Such a 'space' had also a public function.
The 95 Theses treat the practice of indulgences.
On this Halloween day, Martin Luther posted his thesis on the church door. I don't think he was in 'costume' or 'character' when he did so.