What is the expiation for breaking a covenant and an oath regarding masturbation, and how many days must be fasted as expiation for committing it during the daytime in Ramadan?

You must engage in sincere repentance and abstain from this forbidden act. Seek help through remembrance of Allah (dhikr), supplication (du'a), fasting, associating with righteous people, and keeping away from causes of corruption.

You are required to offer expiation for an oath (kaffarat yamin) for every instance you broke your covenant with Allah. This involves feeding ten poor persons, or clothing them, or freeing a slave. If you are unable to do any of these, then you must fast for three consecutive days.

As for the days in Ramadan when you committed the act and semen was discharged, if you were ignorant of the ruling, then you are not required to make up those days. However, if you were aware of the ruling, then you must repent and make up all those days. If you do not know their exact number, then make up a number of days that ensures certainty or a strong presumption of clearing your obligation.