What is the ruling on a woman wearing the hijab only during Ramadan?

A woman is religiously obligated to wear the hijab throughout the year. Her not wearing it is a grave sin, and it is even graver in Ramadan, based on Ibn Taymiyyah's statement: "Punishment and reward are multiplied for one who is in the Sacred Mosque, and for one who does that in the month of Ramadan, and similar cases."

For her repentance to be accepted, she must abandon the disobedience, wear the hijab, regret what has passed, and firmly resolve not to return to tabarruj (immodest display); as Allah Almighty says: "And He it is who accepts repentance from His servants and pardons sins, and He knows what you do."