He vowed to fast every Monday and Thursday but he did not fast for several years
Question

                Twelve years ago I vowed that if I passed the third year of secondary school with a good grade, I would fast every Monday and Thursday. 

For a long time I have not been regular in fasting, but during the last three months I have become regular in fasting. 

What is the ruling on the days when I did not fast? 

What is the ruling on some of the days when I am invited to lunch at my father’s house or elsewhere – should I fast on those days or not?.
Praise be to Allah.
    Firstly: It should be noted that making vows is makrooh, 
    because of the marfoo’ hadeeth of Abu Hurayrah (may Allaah be pleased with 
    him): “Do not make vows, for vows do not affect the divine will at all, all 
    they do is get something from a stingy person.” Narrated by al-Bukhaari, 
    6609; Muslim, 1640. This version narrated by Muslim. 

    And it was narrated from Ibn ‘Umar (may Allaah be pleased 
    with him) that the Prophet  (peace and blessings of Allaah be upon him) 
    forbade vows and said that they do not do any good, rather they just get 
    something from a stingy person. Narrated by Muslim, 1639. A person may 
    oblige himself to do something by making a vow then not fulfil it, thus he 
    puts pressure on himself and exposes himself to sin.  

    Being negligent with regard to a vow is a serious matter, and 
    there is the fear that the one who does that is included in the verses in 
    which Allaah says (interpretation of the meaning): 

    “And of them are some who made a covenant with Allaah 
    (saying): “If He bestowed on us of His Bounty, we will verily, give Sadaqah 
    (Zakaah and voluntary charity in Allaah’s Cause) and will be certainly among 
    those who are righteous.”

    76. Then when He gave them of His Bounty, they became 
    niggardly [refused to pay the Sadaqah (Zakaah or voluntary charity)], and 
    turned away, averse.

    77. So He punished them by putting hypocrisy into their 
    hearts till the Day whereon they shall meet Him, because they broke that 
    (covenant with Allaah) which they had promised to Him and because they used 
    to tell lies”

    [al-Tawbah 9:75-77]
Concerning the 
    seriousness of being negligent with regard to vows, see also question no.
    42178. 

    With answer to this particular question, the vow which you 
    made is a vow of obligation, i.e., it is obligatory for you to fulfil it, 
    because the Prophet  (peace and blessings of Allaah be upon him) said: 
    “Whoever vows to obey Allaah, let him obey Him, and whoever vows to disobey 
    Allaah, let him not disobey Him.” Narrated by al-Bukhaari, 6696. 

    Based on this, you have to fulfil your vow. With regard to 
    the days when you did not fast, you have to make them up. As well as that 
    you have to offer kafaarat yameen (expiation for breaking a vow). One 
    expiation is sufficient for all the days when you did not fast. 

    Ibn Qudaamah was asked in al-Mughni (13/648) about a 
    person who vowed to fast every Thursday forever, then he did not fast for 
    several days. He said; Let him make up his vow and offer expiation… If he 
    has missed many days, then he has to offer one expiation for all of them. If 
    he offered expiation then he missed some more days, then he has to offer 
    another expiation. This was stated by Ahmad. 

    Your being invited to lunch is not regarded as an excuse that 
    allows you not to fast and not to fulfil the vow. You can tell them about 
    your vow so that you will have an excuse not to attend. And Allaah knows 
    best. 

    For more information see questions no.
    36800 and
    38934.