Atuendo/atavío

[poli]

Are they synonymns?

[Don José]

Yes:
http://buscon.rae.es/draeI/SrvltConsulta?TIPO_BUS=3&LEMA=atav%C3%ADo

I'd say "atavío" is much less frequent.

[chileno]

And even then, atavío has always been used when describing native attire.

[powerchisper]

There's some subtle difference.

Atuendo : the combination of clothes you are wearing right now.

Atavío : is the same , but normally said when talking about working clothes , or even native attire , as chileno described above.

Hope it helps !!