When a woman who generally veils her face in public is performing Hajj or `Umrah, does she keep on wearing her face veil? If not, then why?

We follow what the Prophet teaches us in this matter: “Do not wear a shirt or pants or a turban or a cloak. If a person does not have a pair of sandals, he may then wear leather socks, but he must cut them so that they come below his ankles. Do not wear anything dyed with saffron or warse. A woman should wear neither a veil nor gloves.” [Sahîh al-Bukhârî (1707)] 

The woman performing Hajj (pilgrimage) or `Umrah may not wear a veil over her face while in the state of ihrâm. Neither may she wear gloves. 

However, if force of habit causes her put on either a veil or gloves by mistake or due to forgetfulness, she is not obligated to sacrifice a sheep. 

When she completes her observation of ihrâm for her Hajj or `umrah, she can wear whatever she likes. 

If the woman can be seen by strangers, she may temporarily conceal her face from them in some way other than by using a veil. 

She has to cover all the rest of her body whether she is in prayer, performing `umrah, or engaged in something else. 

The mother of the believers, `A’ishah, said: “Men would pass by us while we were with the Prophet (peace be upon him) engaged in our Hajj rites. We would hide our faces, and when they passed by, we would uncover them.” 

And Allah knows best.

-islamtoday.net