What should a woman who usually wears the veil do during Hajj when she is in the state of Ihraam?
The face veil should not be worn in the state of Ihraam. If there are men around or passing by, she may cover her face, in which case she may cover it by pulling down her jilbaab over her face. This can be done by adding extra material to the jilbaab so there is ample cloth to draw over her face if the need arises.
This is because it is better (and according to Ibn `Uthaimeen, obligatory) for the woman not to reveal her face to strange men.
Manaasik al-Hajj wal-`Umrah wal-Mashroo` Fiz-Ziyaarah of Shaikh Muhammad Ibn Saalih al-`Uthaimeen.