What is the complete story of the revelation of the hijab, and what is the difference between what was revealed concerning Sawda and what was revealed concerning Zaynab bint Jahsh, and in what year did each of them occur?

In summary, the hijab (definition) was made obligatory in the story of Zaynab in the fifth year of the Hijra. 'Umar wanted the figures of the Prophet's wives to be veiled by houses or litters, so Allah permitted them to do that when needed. Some scholars are of the opinion that this was obligatory upon them except for necessity, such as relieving themselves. 'Umar had agreements in the revelation of the verse of hijab. They had situations in covering themselves when relieving themselves, including covering themselves with darkness, then the revelation of the hijab, then covering themselves with clothing, then taking kennels in the houses. The story of Zaynab and the story of Sawdah were the reason for the revelation of the verse of hijab. They were permitted to go out for their needs. The obligation of hijab, specifically the face and hands, was exclusive to the wives of the Prophet, peace and blessings be upon him. They used to travel for Hajj and other purposes, and from the necessities of that was circumambulation and the Sa'i. 'Umar was keen on exaggerating in the veiling of the Mothers of the Believers, so that they would not show their figures at all. The Prophet, peace and blessings be upon him, was following the custom of the Arabs in abandoning the hijab, until he was commanded to do so.