Most Japanese are Shinto or Hindu, and therefore do not celebrate Christmas. Therefore, they probably dress the same as most other days.
on Christmas people don't really have an attire, as they dress up very formally
Dress is what is meant by attire. You may receive receive an invitation specifying formal dress. That would mean formal attire. It is what you are wearing.
a dress
causal attire
The beautiful dress
The Eldar Harlequins dress in clownish attire.
enrobe
The word attire refers to clothing, formal or dressy in particular.
kimomo
Business attire
The dress code for the Union League Club of Chicago is business casual attire in general. Special events however require higher forms of attires, such as business dress attire.
Evening attire is nice clothing. For men, evening attire will be a nice tailored suit. For women, its a nice evening dress, usually black.