Abu Dawud

Benefits and Issues:
➊ The majority of scholars from both the early and later generations (salaf and khalaf) hold the view that ‘Ashura refers to the tenth day of Muharram. However, it is narrated from Ibn Abbas (radi Allahu anhu) that by this is meant the ninth of Muharram.
And its explanation is given as follows: The Arabs had a custom that when they brought camels to water after grazing them in the pasture, if they had grazed for two days after watering and came the next day, they would say (wardana arba‘an) “we came on the fourth day” or “we watered on the fourth day,” even though in reality it was the third day.
Similarly, after grazing for three days, if they returned the next day, they would say (wardana khamsan) “we came on the fifth day” or “we watered on the fifth day,” even though in reality it was the fourth day.
In this manner, Ibn Abbas (radi Allahu anhu) considered ‘Ashura to be the ninth day.
(Khattabi) However, this explanation is less preferred; the preferred meaning will be mentioned ahead.

➋ The meaning of this blessed statement is that we should also fast on the ninth day.
In light of other narrations, this is the correct meaning.
In Musnad Ahmad, it is narrated from Ibn Abbas (radi Allahu anhu) in a marfu‘ (attributed to the Prophet sallallahu alayhi wa sallam) narration that the Prophet sallallahu alayhi wa sallam said: “Oppose the Jews; fast a day before or a day after them.”
(Musnad Ahmad: 1/241) ‘Ata’ narrates from Ibn Abbas (radi Allahu anhu) that: “Fast on the ninth and tenth, and oppose the Jews.”
(Al-Bayhaqi: 4/287) From these narrations, it is understood that in the forthcoming narration (2446), when Ibn Abbas (radi Allahu anhu) says: “When you see the crescent of Muharram, count the days, and when the ninth comes, fast,”
by this he does not mean that ‘Ashura is the ninth day, but rather his intent is that before the tenth, also fast on the ninth day.
(Benefits of Imam Ibn al-Qayyim rahimahullah)