Is it permissible for me to take a vow to fast a day as an expression of gratitude to Allah, the Exalted, if my wish is fulfilled, and, at the same time, intend this fasting to be a voluntary act of worship so that Allah, the Exalted, would accept my supplication and fulfill my wish? Should I fast that day before my wish is fulfilled so that it would be considered a voluntary act of worship given that offering voluntary acts is beneficial in terms of having one's wishes fulfilled?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.

A vow is defined as a commitment of an adult Muslim who is competent for religious assignment to do an act of worship that was not already obligatory on him before taking the vow, even if the vow is conditional upon the occurrence of a certain thing. In this aspect, the vow is different from voluntary acts of worship. If you have a wish and hope that Allaah, the Exalted, will fulfill it for you, then it is permissible for you to take a vow to fast a day or some days if your wish is fulfilled. When the thing, upon which you conditioned your vow, takes place, i.e. your wish is fulfilled, then you are obliged to fulfill your vow. If you fast after the fulfillment of your wish or give something in charity as an expression of gratitude to Allaah, the Exalted, for His grace, there is no harm in that. If your wish is not fulfilled, you are not obliged to do anything in this regard.

It is better for you to supplicate Allaah, the Exalted, so much so as to have your wish fulfilled. You may also draw closer to Allaah Almighty by carrying out as much voluntary acts of worship, such as fasting and charitable acts, as you can. This is because, according to the views of some scholars, taking a conditional vow is disliked. Nevertheless, the vow-taker is obliged to fulfill his conditional vow. Such a vow is a means whereby (something) is extracted from the miserly person. The Maaliki scholar Khaleel said, "There are different scholarly opinions regarding considering the conditional vow disliked." [Al-Mukhtasar]

Allaah Knows best.