Costume Parties: Not Just For Halloween!
In North America, the tradition is simple: every October 31st, if you're attending a party, you're probably going to do so while wearing a costume. "Come as you aren't" is a staple for Halloween celebrations, based around old folklore that dressing in disguise would ward of the evil spirits that walk the earth on Halloween Nowadays, this folklore has more translated to "hey, it's really fun!" - meaning that even if it isn't Halloween, you can capture that sense of fun with any party you throw.
People like dressing up; they like costumes, the fun of it, the enjoyment of getting ready and seeing other guests in their outfits. Costumes are also a great ice breaker for parties, as being dressed somewhat ridiculously helps break down people's barriers. The costume party is therefore a natural extension beyond the last day of October, and is applicable all year 'round.
The best way to organize a non-Halloween costume party is to incorporate the dressing up in the theme of the evening. This can be something as simple as theming the costumes around the favorite film of the person whose party it is - James Bond is always good fun, while any Disney theme will certainly give your guests something to think about! Alternatively, you can extend the costume idea to coincide with national holidays outside of Halloween; for example, an 'American Heroes' theme for a Fourth of July party.
Just remember to inform people that the party is a costume (and of any theme, if there is one) on the invitations. No one likes to attend a costume party without being in costume, so make sure everyone knows about the plan well in advance.