What is the ruling on removing the hijab during summer holidays, and what is the expiation for this action?

A woman's hijab (definition) is a sign of her modesty and a symbol of her chastity. It is forbidden for her to remove it in front of non-mahram (definition) men in any place or at any time, due to the Almighty's saying: "O Prophet, tell your wives and your daughters and the women of the believers to bring down over themselves [part] of their outer garments. That is more suitable that they will be known and not be abused. And ever is Allah Forgiving and Merciful." The expiation for this is sincere repentance and a firm resolve not to return to this wrongdoing.