I have read differing positions on this, but I have yet to see any definitive evidence for either positions. It doesn't necessarily have to be Ashura, it could be Arafah or any related fasting that expiates sins. I would like to know whether 'sins' includes all, or only limited to minor ones?
There are some hadiths which say it like this, that fasting on the 10th of Muharram (Ashuraa) expiates the sins of preceding year.
And the ruling of many scholars is that by such actions, about which is said that this action will expiate sins, the minor sins are meant. As far as the major sins are concerned, So for them, the sinner should do Taobah (توبة). And if someone has committed a sin which is related to some other human being (حقوق العباد), the sinner must ask for forgiving that from the person to whom he has done the harm.
Following Hadith is given as an example, and many others could be found like this.
and I seek from Allah that fasting on the day of Ashura may atone for the sins of the preceding year.
The same Hadith is also narrated by Imam Abu Dawood, See it Here.
And Allah Knows the Best.