Bukhari

Hadith Commentary:
Abdullah ibn Wahb himself narrated this hadith to Yahya, or the students of Abdullah ibn Wahb narrated it to him; in both ways, the narration of the hadith is authentic.

Imam Bukhari rahimahullah did not mention in this chapter those ahadith which encourage fasting on the Day of Arafah, whereas he narrated the hadith which establishes that the Prophet sallallahu alayhi wa sallam broke his fast on Arafah.
This is because those ahadith may not have met his conditions of authenticity.

However, Imam Muslim rahimahullah has narrated from Abu Qatadah radi Allahu anhu that the Messenger of Allah sallallahu alayhi wa sallam said: Fasting on the Day of Arafah is an expiation for the sins of the previous year and the coming year. And some have said that the pilgrim (haji) should not fast on the Day of Arafah,
out of concern that he may become weak and thus be hindered in performing the rites of Hajj.

In this way, reconciliation is achieved between the ahadith of the chapter and those ahadith (regarding fasting on Arafah).
(Wahidi)

Hadith Commentary:
(1)
From these narrations, it is understood that the pilgrims (hujjaj) should not fast on the Day of Arafah in the plain of Arafat on the 9th of Dhu al-Hijjah.
It is narrated from Abu Hurairah (radi Allahu anhu) that the Messenger of Allah (sallallahu alayhi wa sallam) forbade fasting on the Day of Arafah in the plain of Arafat.
(Musnad Ahmad: 304/2)
The wisdom mentioned for not fasting on this day is that fasting in the plain of Arafat may cause a person to become weak.
It is possible that due to this, he may become unable to engage in supplication (du'a), remembrance (dhikr), and other virtuous acts there.
And Allah knows best.

(2)
For those other than the pilgrims, fasting on this day is recommended (mustahabb). Thus, it is narrated from Abu Qatadah (radi Allahu anhu) that the Messenger of Allah (sallallahu alayhi wa sallam) said:
"The fast of the Day of Arafah expiates the sins of two years:
one past year and one coming year."
(Sahih Muslim, Kitab al-Siyam, Hadith: 2746(1162))

(3)
Some contemporaries are of the opinion that for non-pilgrims, fasting on the Day of Arafah is recommended, even if it happens to be the 8th of Dhu al-Hijjah in our region. This opinion, for several reasons, is questionable.
➊ Not every person has the ability to determine the Day of Arafah, so he should be obligated to fast on the 9th of Dhu al-Hijjah according to his own local calculation.
➋ In some places, there are differences in dates; for example, in our region (the Indian subcontinent), the Day of Arafah may fall on the 8th, while in some Western countries, the Day of Arafah could be on the 10th, in which case it would be Eid there, and fasting on Eid is prohibited. Therefore, linking the fast to the Day of Arafah in Saudi Arabia is questionable.
In our view, the preferred opinion is that we should rely on our own local sighting of the moon; whichever day is the 9th of Dhu al-Hijjah according to our calculation, we should fast on that day, even if it is Eid in Saudi Arabia.
And Allah knows best.

Hadith Commentary: Benefits and Issues: On the day of Arafah, since the pilgrims go from Mina to Arafat, there they combine the Zuhr and Asr prayers, and the Imam delivers a sermon. Then, until the evening, it is obligatory to remain in the plain of Arafat for supplication and seeking forgiveness, and as soon as the sun sets, one must return towards Muzdalifah. Due to the performance of these tasks, fasting on the day of Arafah becomes difficult and burdensome for the pilgrim; therefore, it is not preferable for pilgrims to fast on the day of Arafah.

For this reason, the Prophet sallallahu alayhi wa sallam, for the purpose of teaching the Ummah, on the day of Arafah—while he was in the plain of Arafat, seated on his camel and observing the standing—drank milk in front of everyone so that all could see that today the Prophet sallallahu alayhi wa sallam was not fasting. The milk was sent after mutual consultation between the two sisters, Umm al-Fadl radi Allahu anha and Maymunah radi Allahu anha, and Ibn Abbas radi Allahu anhu took it; therefore, its attribution can be made to both of them. Umair radi Allahu anhu was the mawla (freed slave) of Ibn Abbas radi Allahu anhu’s mother, Umm al-Fadl radi Allahu anha, but he always remained with Ibn Abbas radi Allahu anhu and was his student and trustworthy companion; thus, he was also referred to as Mawla Ibn Abbas radi Allahu anhu. According to Imam Abu Hanifah rahimahullah, Imam Malik rahimahullah, Imam Shafi’i rahimahullah, and the majority of scholars, it is better for pilgrims not to fast on the day of Arafah.