What is meant by the white and black thread mentioned in the verse on fasting
Question

                What is meant in the verse in which Allaah says “and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night)”?.
Praise be to Allah.Allaah, may 
    He be exalted, says (interpretation of the meaning): 
“It is 
    made lawful for you to have sexual relations with your wives on the night of 
    As‑Sawm (the fasts). They are Libaas [i.e. body-cover, or screen, or Sakan 
    (i.e. you enjoy the pleasure of living with them)] for you and you are the 
    same for them. Allaah knows that you used to deceive yourselves, so He 
    turned to you (accepted your repentance) and forgave you. So now have sexual 
    relations with them and seek that which Allaah has ordained for you 
    (offspring), and eat and drink until the white thread (light) of dawn 
    appears to you distinct from the black thread (darkness of night), then 
    complete your Sawm (fast) till the nightfall”

    [al-Baqarah 2:187]
What this 
    means is that Allaah has allowed those who are fasting to eat and drink at 
    night until he is certain that dawn has come. What is meant by the white 
    thread is the day, and the black thread is the night. 
Al-Haafiz 
    said: What the verse means is until the whiteness of day can be 
    distinguished from the blackness of night. This happens when the true dawn 
    appears. 
The words 
    “of dawn” serve to explain what is meant by “the white thread” and there is 
    no need to explain what is meant by the black thread because explaining the 
    one makes it clear what is meant by the other.  End quote. 
Some of the 
    Sahaabah (may Allaah be pleased with them) understood the verse in a manner 
    other than what it means. They thought that what was meant was a thread in 
    the real sense. One of them would put two threads, one white and one black, 
    beneath his pillow or tie them to his foot, and he would carry on eating 
    until he could tell them apart. They misunderstood it because at first 
    Allaah revealed the verse without the words “of dawn”, then after a while 
    (some scholars say that it was after a year), Allaah revealed the verse with 
    the words “of dawn”. Then they realized that what was meant by the white 
    thread was the light of dawn (the day) and what was meant by the black 
    thread was night. 
Al-Bukhaari 
    (1917) and Muslim (1091) narrated that Sahl ibn Sa’d said: The verse “and 
    eat and drink until the white thread appears to you distinct from the black 
    thread” [al-Baqarah 2:187] was revealed, and the words “of dawn” were 
    not revealed. When people wanted to fast, one of them would tie a white 
    thread and a black thread to his foot, then he would carry on eating until 
    he could tell them apart when he looked at them. Then after that Allaah 
    revealed the words, “of dawn,” 
    then they realized that what was meant by that was night and day. 
The Sahaabah 
    interpreted the word thread according to its apparent meaning, as they 
    understood it, then when the words “of dawn” were revealed, they understood 
    what was meant. 
Al-Bukhaari 
    (1916) narrated that ‘Adiyy ibn Haatim (may Allaah be pleased with him) 
    said: “When the words ‘until the white thread (light) of dawn appears to 
    you distinct from the black thread (darkness of night)’ [al-Baqarah 2:187 – 
    interpretation of the meaning] were revealed, I took a black rope and a 
    white rope and put them beneath my pillow, and I started to look at them 
    during the night, but I could not see any distinction between them. The next 
    morning I went to the Messenger of Allaah (peace and blessings of Allaah be 
    upon him) and told him about that. He said, ‘Rather that is the blackness of 
    the night and the whiteness of the day.’”  
According to 
    a report narrated by al-Bukhaari (4510) he said: “You must be huge.” 
    According to another report (4509) he said: “Your pillow must be very wide, 
    if the white thread and the black thread are beneath your pillow.” And 
    according to another report: (4510) “The back of your head must be very 
    wide.” 
The story of 
    ‘Adiyy ibn Haatim occurred after Allaah had revealed the words “of dawn”, 
    i.e., after the hadeeth of Sahl quoted above. Some of the scholars explained 
    Adiyy’s mistake of misunderstanding the verse even though the words “of 
    dawn” had been revealed, by noting that the hadeeth of Sahl had not reached 
    ‘Adiyy’, or that in the dialect of his people the words “white thread” and 
    “black thread” were not used to refer to day and night. 
Hence Ibn 
    Hibbaan commented on the hadeeth of ‘Adiyy by noting that “the language of 
    the Arabs varies,” and pointed out that in the language of ‘Adiyy, the 
    darkness of night and the light of day were not described as a black thread 
    and white thread. Al-Qurtubi said: The hadeeth of ‘Adiyy is later than the 
    hadeeth of Sahl. It is as if ‘Adiyy had not heard what was said in the 
    hadeeth of Sahl, rather he only heard the verse and understood it as it 
    sounded to him. End quote. 
Al-Haafiz 
    said: As for ‘Adiyy, it is as if in the language of his people they did not 
    refer to the dawn as a thread, or he forgot the words “of dawn” until the 
    Prophet  (peace and blessings of Allaah be upon him) reminded him of 
    that. 
Al-Nawawi 
    said, following al-Qaadi ‘Iyaad: Rather the black and white threads were 
    interpreted according to their apparent meaning by some Bedouin who did not 
    have much knowledge, such as the men from whom Sahl narrated, and some of 
    those in whose dialect the dawn was not referred to as a thread, such as 
    ‘Adiyy. End quote. 
With regard 
    to the words of the Prophet  (peace and blessings of Allaah be upon 
    him), “The back of your head must be very wide,” some of them claimed that 
    what the Prophet  (peace and blessings of Allaah be upon him) meant by 
    that was that he was unintelligent. They claimed that if the back of the 
    head is wide, this is a sign of stupidity, and they wrote poetry about that. 
    But many of the scholars rejected this, such as al-Qurtubi, al-Qaadi ‘Iyaad 
    and al-Nawawi. 
Al-Qurtubi 
    said: 
Some people 
    interpreted this as a criticism of him for that misunderstanding. It is as 
    if they understood it as an accusation of ignorance and lack of 
    understanding, but it is not as they say. Rather he meant – and Allaah knows 
    best – if your pillow covers the two threads to which Allaah referred then 
    it must be wide indeed. Hence immediately after that he said:  “Rather that 
    is the blackness of the night and the whiteness of the day.”  It is as if he 
    said: How can they fit under your pillow? And the words “The back of your 
    head must be very wide” mean that the pillow which covers the night and day 
    cannot be rested upon except by a head that is very wide. End quote. 
Al-Qaadi 
    said: What this means is that if you put under your pillow the two threads 
    that Allaah meant, namely the night and day, and your pillow covered them, 
    then it must be very wide. This is what is meant in the other report 
    narrated in Saheeh al-Bukhaari – “The back of your head must be very 
    wide” – and in the other report: “You must be huge.” 
See: Fath 
    al-Baari, commentary on hadeeth no. 1917 and 1916’ Sharh 
    Muslim by al-Nawawi, hadeeth no. 1090, 1091 
Among the 
    rulings that are derived from this verse is the ruling that if a person is 
    unsure whether dawn has broken, he may carry on eating and drinking until he 
    is sure of it, because Allaah says “until the white thread (light) of 
    dawn appears to you distinct from the black thread”. 
‘Abd 
    al-Razzaaq narrated that Ibn ‘Abbaas said: Allaah has permitted you to eat 
    and drink so long as you are not sure. Al-Haafiz said: its isnaad is 
    saheeh. 
Ibn Abi 
    Shaybah narrated that Abu’l-Duha said: A man asked Ibn ‘Abbaas about sahoor 
    and Ibn ‘Abbaas said: Eat so long as you are uncertain, until you are no 
    longer uncertain. This is the view of the majority of scholars. 
Shaykh Ibn 
    ‘Uthaymeen said in al-Sharh al-Mumti’ (6/247): 
If a person 
    is not fasting, and he is not sure whether dawn has broken, his fast is 
    valid, because Allaah says (interpretation of the meaning):
“So now 
    have sexual relations with them and seek that which Allaah has ordained for 
    you (offspring), and eat and drink until the white thread (light) of dawn 
    appears to you distinct from the black thread (darkness of night), then 
    complete your Sawm (fast) till the nightfall”

    [al-Baqarah 2:187]
The opposite 
    of certainty is uncertainty and doubt; so long as we are not certain, we may 
    eat and drink. This issue falls into five categories: 

    1-If he is certain that dawn has 
    not come, such as if dawn comes at five o’clock 
    and he eats and drinks at four-thirty, his fast is valid.

    2-If he is certain that dawn has 
    come, such as if he eats at five-thirty, then his fast is invalid.

    3-If he eats and he is unsure 
    whether dawn has come or not, but he thinks it most likely that it has not 
    come, then his fast is valid.

    4-If he eats and drinks, and he 
    thinks it most likely that dawn has come, then his fast is valid.

    5-If he eats and drinks and is 
    unsure, then his fast is valid. End quote.
And Allaah knows 
    best.