I will be there. There is still no information about the timing.
Something is strange though : they will give the hat "blinking evil red", and I think this hat is more powerful than the usual Wzard Hat (for a wizard). I don't remember the exact characteristics of this new hat, and it is not referenced in the wiki. Next time someone see a GM with it, could he/she report what those characteristics are ?
It is my understanding that the GM event will be day-long, in order to give as many players as possible an opportunity to obtain a nice prize.
As for the Mask of Blinking Evil, the green-eyed one was available from last year's special Halloween Quest. Here is information about it:
Mask of Blinking Evil (Green), 2270 Made of a strange, animated stone, this mask seems to cling to your face. Defense +13 / M. Attack +10 / M. Defense +5 Weight: 50 g
Since last Halloween, there have been a few GM events where the masks were given as prizes. In those events the winners got to choose the color they preferred. The mask comes in five eye colors: green, red, blue, yellow, orange, and pink.