The Prophet ﷺ said fasting the Day of ʿArafah results in the expiation of sins of the year that has passed, and the year that is to come. He ﷺ also said that fasting the Day of ʿĀshūrāʾ results in the expiation of the sins of the year that has passed [Muslim (1162 (197))].

Does this expiation of sins include major and minor sins, or just minor sins?

Sh. Muḥammad b. Ṣāliḥ al-ʿUthaymīn رَحِمَهُ اللهُ explains that the apparent meaning of the ḥadīth indicates the expiation of both major and minor sins. This is because the Prophet ﷺ mentioned it in an unqualified manner and did not elaborate further, and whatever the Prophet ﷺ left unrestricted, it is taken to be unrestricted. This is why some scholars took the view, that the expiation of sins, refers to both minor and major because the Prophet ﷺ left it unrestricted, and did not elaborate further.

Most of the scholars take the view the expiation is only of minor sins. As for major sins then they must be accompanied with repentance. They support their view by arguing that fasting the Day of ʿArafah is not superior to the five daily prayers or Jumuʿah or Ramaḍān, and the Prophet ﷺ said, “The five daily prayers, Jumuʿah to Jumuʿah, and Ramaḍān to Ramaḍān, expiate what is between them as long as the major sins are avoided.” [Muslim (233)]. They say if these great and noble acts of worship, which are from the pillars of Islām, are not strong enough to expiate major sins, then this even more the case with this optional day of fasting. This is the weightier view – that it is qualified, just as the five daily prayers and Ramaḍān to Ramaḍān are qualified.

Source: Sharḥ Bulūgh al-Marām (7: 356).