Halloween is the first cosplay convention that ever was, and the longest running one, but Halloween is more than just that. It's a time for people to connect with the pop culture they love and embody that pop culture. For example, the recent Verizon commercial shows a family dressed up as characters (and more) from Star Wars. What strikes me about that commercial is that for that family Star Wars is real that night and in a way they get to become those characters while they trick and treat (though they do seem more obsessed with Candy than anything else).
Halloween is one of my favorite holidays, and it's also one of the inspirations for my approach to pop culture magic. This isn't surprising because its during Halloween that pop culture comes out in force. People dress up as the characters they love and for an evening embody those characters in one form or another. This occurs across ages, with little children dressing up to go trick and treat, while adults dress up to have fun at a costume party. Now not all of these people intentionally set out to work magic, but Halloween is a night of masks, and as such it can be useful for magical work to explore the idea of taking on a mask.
A mask allows a person to become something else, to invoke a different presence into his/her life. The mask isn't a permanent change in identity, but rather is a temporary change that allows the person to access what the mask represents. And what the mask represents is a chance to let go and allow yourself to connect with something that isn't you, that is different from your usual identity. Of course there are potential dangers when you do this without the right constraints, and I think that one of the constraints that is present in Halloween is the idea that it's all make believe. It's a useful constraint for people who aren't magicians, but for someone who practices magic, putting on the mask is never make believe. Putting on the mask is a connection with the character, entity, deity, etc that the mask represents.