She vowed that if the arguments with her husband ceased, she would fast on Thursdays forever
Question

                There is a woman who was having arguments with her husband, and she vowed that she would fast every Thursday for the rest of her life if these arguments ceased without it resulting in divorce. The arguments ceased and she began to fast on Thursdays, but Allaah decreed that the same argument should arise again, when they both went to her father’s house, and the husband issued a divorce in a moment of frustration and anger. The woman continued to fast on Thursdays and after a while they reconciled and the arguments ceased. Does she still have to fast every Thursday for the rest of her life, knowing that she is also fasting every Monday for the rest of her life in fulfillment of another vow?.
Praise be to Allah.This is what 
    the scholars called a conditional vow (nadhr mu’allaq), as the vow is 
    subject to the condition that a certain thing happen. The ruling on this is 
    that if the thing vowed is an act of worship and the thing stated as the 
    condition comes to pass, it become obligatory to fulfil the vow, because the 
    Prophet  (peace and blessings of Allaah be upon him) said:  “Whoever 
    vows to obey Allaah then let him obey Him, and whoever vows to disobey 
    Allaah let him not disobey Him.” Narrated by al-Bukhaari, 6318. 
Ibn Qudaamah 
    (may Allaah have mercy on him) said concerning vows to do acts of worship: 
    “These are of three types, one of which is doing the acts of worship in 
    return for a blessing bestowed or a harm warded off, such as if a person 
    says, ‘If Allaah heals me, then I will fast one month for Allaah.’ So this 
    act of worship becomes binding because it is obligatory in principle in 
    sharee’ah, like fasting, prayer, charity and Hajj, and it is obligatory to 
    fulfil this vow according to scholarly consensus.”  
Al-Mughni, 
    13/622 
The 
    condition in the case of this vow is the cessation of arguments between this 
    woman and her husband, without divorce. 
One of the 
    following two scenarios must apply in this woman’s case. 
Either she 
    wanted a specific argument that was taking place at the time of the vow to 
    cease without divorce, in which case she is obliged to fulfil the vow 
    because the condition was met, which is that the argument ceased without 
    divorce, and the divorce that took place after that does not matter because 
    that had to do with a different argument,  
Or she 
    wanted the arguments that were taking place between her and her husband for 
    a particular reason to cease altogether, so that they would not happen 
    again. In this case she is not obliged to observe these fasts, because the 
    condition was not fulfilled, as the arguments happened again and divorce 
    occurred as a result. Perhaps what she says in her question, “but Allaah 
    decreed that the same argument should arise again”, is indicative of that. 
This woman 
    should define what she meant, and then do whatever she is obliged to do 
    according to sharee’ah. 
It should be 
    noted that vows are makrooh in principle, because of the report narrated by 
    al-Bukhaari (6608) and Muslim (1639) from Ibn ‘Umar (may Allaah be pleased 
    with him), that the Prophet  (peace and blessings of Allaah be upon him) 
    forbade vows and said: “They do not change anything, they only make a stingy 
    person give (in charity).” 
Al-Maaziri said: It may 
    be understood that the reason for them not being allowed is that a vow 
    becomes binding, and he may do it under compulsion, without enthusiasm. Or 
    it may be understood that the reason is that he may offer the sacrifice that 
    he promised in his vow in return for the things that he asked for, and thus 
    his reward will be reduced. The point of worship is that it should be purely 
    for the sake of Allaah. Al-Qaadi ‘Iyaad said: It may be understood that the 
    prohibition is because some ignorant people may think that the vow changes 
    the divine decree and prevents the divine decree from being carried out, so 
    it was forbidden lest some ignorant people believe that. The context of the 
    hadeeth supports that. And Allaah knows best.  
From 
    Sharh Muslim by al-Nawawi. 
The believer 
    should avoid doing that which the Prophet  (peace and blessings of 
    Allaah be upon him) disallowed. Whoever wants to obey or worship Allaah, let him do so without making vows. 
And Allaah knows best.