Shaykh Saalih al-Fowzaan said:
Yes, it expiates…it expiates [the sins of] the year which has passed. Meaning that which it (the year) contained of sins. That which it (the year) contained of minor sins. The expiation is for the minor sins.
As for the major sins, then they are not expiated except by repentance.
As for the minor sins, then Allaah expiates them by way of many things (different acts of worship),…yes.
From among them is fasting on the day of ‘Aashooraa…yes.