What is the ruling on someone who swore: "By God, I will do such-and-such" on Wednesday, then forgot to do it on Wednesday; is it obligatory to do it after Wednesday?

If someone swears to do something on a specific day (like Wednesday) but forgets and does not do it, then his oath is not fulfilled, and doing it on another day is not sufficient; because the action did not occur at its appointed time. Scholars have differed on whether expiation is incumbent upon this swearer, and it is more prudent to offer expiation for an oath to avoid the difference of opinion.