The scholars differed concerning one who starts to observe a naafil fast: is he obliged to complete it or not? There are two views: 

1 – That he does not have to complete a naafil fast. This is the view of the Shaafa’is and Hanbalis, who quoted the following evidence: 

(i)It was narrated that ‘Aa’ishah the Mother of the Believers said: “The Prophet (peace and blessings of Allaah be upon him) entered upon me one day and said: ‘Do you have anything (to eat)?’ We said: ‘No.’ he said: ‘Then I am fasting.’ Then he came to us another day and we said: ‘O Messenger of Allaah, we have been given some hays (a kind of meal),’ and he said, ‘Show it to me, for I started the day fasting.’ Then he ate.” (Narrated by Muslim, no. 1154).

(ii)It was narrated that Abu Juhayfah said: “… Abu’l-Darda’ came and he made some food for him – i.e., for Salmaan. He said: ‘Eat, for I am fasting.’ Salmaan said: ‘I will not eat until you eat.’ So he ate… and Salmaan said to him: ‘Your Lord has rights over you, your soul has rights over you, and your family has rights over you, so give each of them their rights.’ Then he came to the Prophet (peace and blessings of Allaah be upon him) and told him about that, and the Prophet (peace and blessings of Allaah be upon him) said: ‘Salmaan is right.’” (Narrated by al-Bukhaari, 1968).

(iii)It was narrated that Abu Sa’eed al-Khudri (may Allaah be pleased with him) said: “I made some food for the Prophet (peace and blessings of Allaah be upon him), and when it was served a man said: ‘I am fasting.’ The Messenger of Allaah (peace and blessings of Allaah be upon him) said: ‘Your brother has invited you (to eat) and has gone to some effort for your sake, so break your fast and fast another day in its stead if you wish.’” (Narrated by al-Daaraqutni, no. 24; classed as hasan by al-Haafiz in al-Fath, 4/210).

2 – The second view is that naafil fasts should be completed, and if a person breaks such a fast he has to make it up. This is the view of the Hanafis who quoted the following evidence that such fasts have to be made up: 

(i)It was narrated that ‘Aa’ishah (may Allaah be pleased with her) said: “Hafsah and I were given a gift of food and we were fasting, so we broke our fast. Then the Messenger of Allaah (peace and blessings of Allaah be upon him) entered and we said to him: ‘O Messenger of Allaah, we were given this gift and we wanted to eat so we broke our fast.’ The Messenger of Allaah (peace and blessings of Allaah be upon him) said: ‘It doesn’t matter; fast another day in its stead.’” (Narrated by Abu Dawood, 2457; al-Tirmidhi, 735. its insaad includes Zameel, of whom it says in al-Taqreeb: he is majhool (unknown). It was classed as da’eef by al-Nawawi in al-Majmoo’, 6/396 and by Ibn al-Qayyim in Zaad al-Ma’aad, 2/84. it was also classed as da’eef by al-Albaani.

(ii)With regard to the hadeeth of ‘Aa’ishah quoted above from Muslim, some of them add the words: “… ‘I started the day fasting,’ and he ate and said: ‘I will fast another day in its stead.’”

It should be noted that al-Nasaa’i classed this addition as da’eef (weak) and said: This is wrong. It was also classed as da’eef by al-Daaraqutni and al-Bayhaqi. The first view is the one that is more likely to be correct, because its evidence is stronger. And it is supported by the report narrated from Umm Haani’ (may Allaah be pleased with her) who said: “O Messenger of Allaah, I broke my fast and I was fasting.” He said to her: “Were you making up a day you had missed?” She said: “No.” He said: “Then it does not matter if it was a voluntary fast.” Narrated by Abu Dawood, no. 2456; classed as saheeh by al-Albaani. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: If a person is observing a naafil fast and something happens that means he has to break his fast, then he should break his fast. This is what was narrated from the Prophet (peace and blessings of Allaah be upon him), that he came to the Mother of the Believers ‘Aa’ishah (may Allaah be pleased with her) and said: “Do you have anything to eat?’ She said: “We have been given some hays (a kind of meal). He said: “Show it to me, for I started the day fasting.” Then he (peace and blessings of Allaah be upon him) ate some of it. This has to do with naafil fasts, not those that are obligatory. 

Majmoo’ al-Fataawa, 20. 

Based on this, you do not have to make up the day when you broke your fast, because the one who is observing a voluntary fast is in charge of himself; rather he should complete six days of Shawwaal. And Allaah knows best.