What should I do if I vowed to fast a day after committing a sin, and I committed the sin in Ramadan? Do I fast after Eid, or what?

This vow is a vow of contention, and it entails an expiation for breaking an oath (kaffarat yamin) according to the most preponderant opinion, or its maker is given the choice between expiation and fulfilling the vow. Disobedience in Ramadan is more severe, and a Muslim should hasten to perform good deeds in this month, as the Prophet, peace and blessings be upon him, said: "May he be humiliated, a man upon whom Ramadan enters and then departs before he is forgiven." If the disobedience was masturbation in Ramadan, then the fast is invalid and must be made up. The expiation for the aforementioned vow is the expiation for breaking an oath, whether the disobedience occurred in Ramadan or otherwise.