Praise be to Allah.
The Muslim should be careful with regard to the issue of vows, because making a vow to Allah to do something or to refrain from something is not a light matter. The one who makes a vow is enjoined to fulfil it and is subject to a threat of punishment if he breaks that vow. Allah, may He be exalted, says (interpretation of the meaning):
“And fulfil (every) covenant. Verily! the covenant, will be questioned about”
“And of them are some who made a covenant with Allah (saying): ‘If He bestowed on us of His Bounty, we will verily, give Sadaqah (Zakat and voluntary charity in Allah’s Cause) and will be certainly among those who are righteous.’
Then when He gave them of His Bounty, they became niggardly (refused to pay the Sadaqah (Zakat or voluntary charity)), and turned away, averse.
So He punished them by putting hypocrisy into their hearts till the Day whereon they shall meet Him, because they broke that (covenant with Allah) which they had promised Him and because they used to tell lies”
Shaykh as-Sa‘di (may Allah have mercy on him) said: So let the believer beware of acquiring this abhorrent characteristic, whereby he makes a promise or pledge to his Lord that if he gets what he is seeking, he will do such and such, then he does not fulfil that, for perhaps Allah will punish him with hypocrisy, as He punished these people.
End quote from Tayseer al-Kareem ar-Rahmaan fi Tafseer Kalaam al-Mannaan (p. 345).
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (9/401): Ahmad said: Vows are a very serious matter, as is indicated in ten places in the Book of Allah: “And fulfil (every) covenant. Verily! the covenant, will be questioned about” [al-Isra’ 17:34]. So he should seek to draw closer to Allah, may He be exalted, if he makes a vow then breaks it, by whatever means he can. ‘Aa’ishah manumitted forty slaves, then she wept until her headcover became wet and lamented breaking her oath. End quote.
The ruling on promises made to Allah is the same as the ruling on vows (nadhr), if one who is accountable intended thereby to compel himself to do some act of worship, such as if he said: I promise Allah that I will pray two rak‘ahs, or I promise Allah that I will complete the Qur’an every month.
But if his intention by making this promise was not to do an act of worship – rather it was to make himself do something or refrain from doing something – then this comes under the ruling on oaths (yameen). For example, if he says: “I promise Allah that I will complete the Qur’an once every month if I speak to So and so,” then this act of worship, which is completing the Qur’an every month, is not the point of his promise; rather the purpose of his promise is to make himself not speak to that person.
Abu Bakr al-Jassaas (may Allah have mercy on him) said, commenting on the verse in which Allah, may He be exalted, says (interpretation of the meaning), “And of them are some who made a covenant with Allah (saying): ‘If He bestowed on us of His Bounty…” [at-Tawbah 9:77]: this indicates that the one who makes a vow (nadhr) to do an act of worship is obliged to fulfil it, because the covenant is a vow.
End quote from Ahkaam al-Qur’an (4/350).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: If a person says: I promise Allah that I will do Hajj this year, this is a vow (nadhr) and pledge (‘ahd) and an oath (yameen). But if he says: I shall not speak to Zayd, then this is an oath (yameen) and a pledge (‘ahd), but it is not a vow (nadhr). So if the oath (yameen) comes under the same heading as a vow (nadhr), which is that it is a commitment to do an act of worship to draw closer to Allah, then it becomes binding upon him to fulfil it.
End quote from al-Mustadrak ‘ala Majmoo‘ al-Fataawa (5/144).
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: A vow (nadhr) may be made verbally, and there is no specific wording for it. Rather anything that implies committing oneself to something is a vow (nadhr), regardless of whether one says “I promise Allah” or “I vow to Allah” and other similar phrases that imply committing oneself, such as saying: “I swear to Allah that I shall do such and such,” even if that is without saying specific words such as vow (nadhr) or pledge (‘ahd). End quote from ash-Sharh al-Mumti‘ (15/207)
Based on that, the promise to which you committed yourself does indeed come under the heading of a vow to do an act of obedience or worship, and a vow to do an act of obedience or worship must be fulfilled, because the Prophet (blessings and peace of Allah be upon him) said: “Whoever vows to do an act of obedience to Allah, let him do it.” Narrated by al-Bukhaari (6696).
If it is proven that you did not complete the Qur’an in some months, then you must make up those complete readings, and as well as making them up you must offer expiation for breaking the oath (kafaarat yameen) for each month, because if fulfilment of an oath is delayed from its specific time, then expiation for breaking the oath is due for that.
Al-Mirdaawi (may Allah have mercy on him) said in al-Insaaf (11/141): If a person vows to fast a specific month, then he does not fast it, with no excuse, then he must make it up and offer expiation for breaking his oath – and there is no scholarly dispute on this point. If he did not fast it because he had an excuse, then he must make it up – and there is no scholarly dispute on this point – but there are two views narrated (from Imam Ahmad) as to whether he must offer expiation. Our view is that he must also offer expiation. End quote.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said, concerning a person who vowed to fast ten days of a particular month, then he did not fast those days during that month, but he fasted them during the following month: We say to him: you have to offer expiation for breaking an oath, because your vow included two things: the vow to fast ten days and that this would be done in this particular month. Because he did not do it in that particular month, he must offer expiation for breaking a vow, because he did not fast at the time stated; but as for the number of days, he did fast that number.
End quote from Majmoo‘ Fataawa Ibn ‘Uthaymeen (19/377).
The expiation must be paid from your money; if you do not have any money then you do not have to take the price of the expiation from your family, but it is permissible for you to take it from them if they give it to you. If they do not give you the money with which to offer expiation, and you do not have any money of your own, then it is sufficient for you to fast three days.
From your saying “once every month” two things may be understood:
1.That the vow is ongoing, because the phrase “every month” implies something that is ongoing and continuing. So you must fulfil that vow for the rest of your life.
2.That the month is the timeframe for each completion of the Qur’an. According to that, from the time when you made that vow you must start your reading, so that no month will go by without you completing it, and so on. Every month you have to read the (entire) Qur’an once, at least. And if you read it more than that, there is nothing wrong with that.
And Allah knows best.