Why the eve and day of seventh of Muharram are dedicated to commemorate the martyrdom of Aba Al-Fadl Al-Abbas (peace be upon him)?

The believers used to dedicate the eve and day of the seventh of the sacred month of Muharram to commemorate the martyrdom anniversary of Aba Al-Fadl Al-Abbas (peace be upon him). Some people may ask: Why is that, despite that the martyrdom of Imam Al-Hussayn, his family and companions were all on the tenth of the sacred month of Muharram?
The custom during the first days of the sacred month of Muharram is to dedicate every night to speak about the eminent personalities of Ahl Al-Bayt (peace be upon him) who were martyred on the timeless tragedy of At-Taf, to show their steadfast positions and heroism on the battle that day.
As for choosing the eve of the seventh for Al-Abbas (peace be upon him), it is stated in the history that on the seventh of the sacred month of Muharram, Abu Al-Fadl Al-Abbas (peace be upon him) was anguished to see the children and women of Ahl Al-Bayt distressed because of thirst. He decided then to go and bring water by force, accompanied by thirty horsemen and twenty of infantry, taking with then twenty bladders and all, attacked the siege made by the enemy army on the Euphrates River, Nafi' Bin Hilal Al-Muradi, one of the most faithful companions of Imam Al-Hussayn faced 'Amru Bin Al-Hajjaj Al-Zubaydi, one of the criminals of Karbala battle, and who has been entrusted to guard the Euphrates, so he said to Nafi':
What brought you here?
he said: We came to drink the water that you prohibited us
He replied: You can drink..
Nafi' then replied: I drink and Al-Hussayn and the rest of his companions are thirsty ?
The criminal said: No way that they drink. We are here to prohibit them from the water!
But this, did not stop the heroes from the companions of Imam Al-Hussayn (peace be upon him), and mocked of his words, they broke into the Euphrates to fill their water skins. 'Amru Bin Al-Hajjaj railed in their faces with a detachment of his soldiers, but the hero of Karbala; Aba Al-Fadl (peace be upon them) and Nafi' Bin Hilal faced them and there was a battle between them, but no one was killed on both sides, and the companions of Imam lead by Aba Al-Fadl (peace be upon him) returned to the camps with their full water skins.
So, Abu Al-Fadl (peace be upon him) gave water to the members of Ahl Al-Bayt (peace be upon them) and quenched their thirst, and since he was given the title of "Saqqaa' ", which is one of its most famous titles.