Must I make up the fast of Thursday that I vowed and then forgot for three weeks?

Fulfilling a vow is obligatory when it entails obedience to Allah, whether the vow is absolute or conditional, such as fasting on a specific day. If the person who made the vow forgets a four-rak'ah daily prayer that he vowed, he should make it up whenever he remembers it, without sin or expiation. However, if he deliberately delays it beyond its specified time, he incurs sin and must offer expiation for a broken oath, because it is a timed prayer that was intentionally delayed beyond its prescribed time. The Prophet (peace be upon him) forbade vows because they bring no good, are often difficult to fulfill, and may lead to hypocrisy.