casual dress
Clothing that is not considered or defined as being ceremonial, formal, semi-formal, or business dress, typically comfortable and with a wider range of choice regarding color and dimensions (as of length in shirt sleeves, pant legs, and skirts). I had to wear a suit and tie every day in my last job, but I'm happy my new office lets everyone come in casual dress each day. The party is going to be very informal, so please attend in casual dress.