A Vow Does not Prevent Anything that Has Been Decreed by Allah

Question :

My wife vowed to herself that she would fast for six days every month if her son graduated from primary school, and he did so about a year ago. She started to fast from that. date, but now she regrets it and feels burdened, due to the fact that she is busy with bringing up her children, and her domestic affairs, especially during the summer. So what is the view of Your Eminence regarding this vow? Should she continue to fast, or seek forgiveness from Allah and repent to Him, bearing in mind that she vowed to fast for six days every month for the term of her life?

Answer:

She must fulfill her vow, according to the words of the Prophet ﷺ :
"Whoever vowed to obey Allah, he should obey Him and whoever vowed to disobey Allah, he should not disobey Him."

And Allah, Most Glorified has praised those who fulfill their vows in His Words:
"They (are those who) fulfil (their) vows, and they fear a Day whose evil will be wide-spreading."

There is no sin upon her if she fasts them separately, if she did not make the intention to fast them consecutively. But if she made the intention to fast them consecutively, she must fast them consecutively.

We ask Allah to help her in that and to magnify her reward. We advise her and other Muslims not to practice making vows, in accordance with the words of the Prophet ﷺ :
"Do not make vows, because a vow does not prevent anything of Allah's Predestination being implemented, it is only a means of extracting something from the miserly.
And Allah is the Granter of success.

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 6 Pages 419-420