My cats would freak out if I tried to dress them up. I once tried to put some reindeer antlers on Forest for a Christmas picture and almost lost an arm in the process. So no costumes for them. (We do joke, though, that Tailer goes as Forest and Forest goes as Tailer.)
I love to decorate and dress up for Halloween. The first year we were at our house I dressed up to receive trick-or-treaters, but unfortunately, we're on a busy street and don't get many. So I don't dress up anymore and I just buy full-sized candy bars for the dozen or so who do come.
My youngest brother's birthday is just before Halloween so sometimes we dress up and do something Halloweenish for his birthday. He also got married last year on Halloween and everyone had to come in costume. He and his wife are gothy types, so I went goth for the occasion. (I must have done a pretty good job of it...my SIL asked if she could have the clothes when I was done with them.