Must I make up for a missed day of fasting from an oath that I don't remember what it was about?

If you are certain of an oath and having broken it, then you must offer expiation even if you do not remember what was sworn upon. If you are certain of an oath and have not broken it, but you have forgotten what you swore upon, then it is permissible for you to offer expiation for it, based on the opinion of the majority of scholars who allow expiation for an oath before breaking it. It must be noted that fasting as an expiation for an oath is only permissible after being unable to feed ten needy persons, or clothe them, or free a slave.