If you have been invited to a fancy dress party or a themed night then you need to allow plenty of time to find an outfit. If you leave it until the last minute you may not be able to get what you want and will often have to pay quite a lot more than if you had ordered it early.
First do some research as to what sorts of outfits you may be able to wear. It could be a super heroes fancy dress for example or maybe a 1950’s night. Once you know the sorts of outfits, try and think about what you would like. It doesn’t have to be silly, you may decide that you still want to look good even if you are dresses up.
IF the theme is an era then you may want to look for quality vintage clothing instead. You may pay a little more than for a fancy dress costume but it will be a lot better made and look more authentic. You can also be quite confident that no one will have exactly the same thing on a you. You may even drop lucky and be able to pick up an outfit from a charity shop that is just perfect for the occasion.