The Noble Shaykh Saalih al-Fawzaan (may Allah have mercy upon him) stated:
This day (the Day of 'Arafah) has within it a tremendous virtue. When he (the Prophet (ﷺ)) was asked about fasting on it he said:
It expiates the previous year and the remaining year.
Meaning the following year.
Fasting (on that day) expiates two years.
This indicates the virtue of fasting the Day of 'Arafah. It expiating the previous year, this is clear because it expiates the evil that's present and that which has taken place. However, it expiating the following year and nothing from it has taken place, this has within it a problem (for many regarding its understanding). How can it expiate that which hasn't taken place? They (the scholars) have said, the meaning of it expiating the remaining year (meaning the following year) is that the person is given the success to keep away from the acts of disobedience in the year that comes after. Or if there does take place from him an act of disobedience, he is given the success to make Tawbah. This is the meaning of the fast expiating the remaining year.
Question: Is the expiation of the sins something for the major sins as well as the minor sins or is it specific for the minor sins?
Answer: The expiation of the sins is specific for the minor sins. As for the major sins, nothing expiates it except for making Tawbah. This is based upon the statement of Allah:
إِنْ تَجْتَنِبُوا كَبَائِرَ مَا تُنْهَوْنَ عَنْهُ نُكَفِّرْ عَنْكُمْ سَيِّئَاتِكُمْ وَنُدْخِلْكُمْ مُدْخَلًا كَرِيمًا[An-Nisa':31]
If you abstain from the major sins that you have been prohibited from We will expiate from you your evil actions.
However, fasting on the Day of 'Arafah is only recommended for those not standing in it. As for the one standing in 'Arafah, then he should not be fasting that day, following the example of the Prophet (ﷺ). Indeed he stood in 'Arafah not fasting on this day.