|In the days of the Flinstones, nobody knew about aurat. But now we do.|
The Quran tells about how to cover aurat. So read the Quran, Surah 24, Ayat 31.
It is not only for Nabi Muhammad's SAW wives as he had directed Asma', the elder sister of Aishah, his wife (see YouTube below). It specifically refers to the tudung, and what it should cover, and how long the cover should be. The Quran is very clear on this matter, and that is why Islam is also referred to Islami Deenah, a perfect religion - it lays down the rules for everything that concerns us humans. We cannot say the Quran is not clear on this matter. We listen, we hear, we know and we abide by the rules made by Allah SWT as found in the Quran. In Akhirat, don't say the Quran in unclear or nobody told you that it is wajib to cover or that it is wearable for certain occasions. Allah SWT will be your judge, so don't play against His rules.
A good write-up about tudung: http://www.islamfortoday.com/niqaabtara.htm
Pesanan Rasulullah tentang menutup aurat