Why does Kane wear a mask one week, and the next week he is without a mask?
This is part of the storyline. This is known in pro-wrestling as kayfabe.
From the beginning of the character's career until around 2003, Kane was masked. He was unmasked for a long period of time (long after I stopped following pro-wrestling) after that.
Last year, he had been unmasked to carry out his role as the "Director of Operations"(1) as part of "The Authority." After WrestleMania XXX, Stephanie McMahon and Kane had a vignette that ultimately led to a remasking and thereafter referring him to as "The Demon Kane."