Fasting in the Second Half of Sha’ban
Question

Is it permissible to fast after halfway through Shabaan? Because I heard that the Prophet (peace and blessings of Allaah be upon him) forbade fasting after halfway through Sha’baan.
Praise be to Allah.Hadiths that prohibit fasting after 15th of Sha’ban
Abu Dawud (3237), al-Tirmidhi (738) and Ibn Majah (1651) narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “When Sha’ban is half over, do not fast.” Classed as sahih by al-Albani in Sahih al-Tirmidhi, 590. 
This hadith indicates that it is not allowed to fast after halfway through Sha’ban, i.e., starting from the sixteenth day of the month. 
Hadiths that allow fasting after 15th of Sha’ban
But there are reports that indicate that it is permissible to fast at this time. For example: 
Al-Bukhari (1914) and Muslim (1082) narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “Do not anticipate Ramadan by fasting one or two days before it begins, but if a man habitually fasts, then let him fast.” 
This indicates that fasting after halfway through Sha’ban is permissible for someone who has the habit of fasting, such as a man who regularly fasts on Mondays and Thursdays, or who fasts alternate days, and the like. 
Al-Bukhari (1970) and Muslim (1156) narrated that ‘Aishah (may Allah be pleased with her) said: “The Messenger of Allah (peace and blessings of Allah be upon him) used to fast all of Sha’ban, he used to fast Sha’ban except a few days.” This version was narrated by Muslim. 
Al-Nawawi said: In the words, “He used to fast all of Sha’ban, he used to fast Sha’ban except a few days” the second phrase explains the first, and indicates that the word “all” means “most of”. 
This hadith indicates that it is permissible to fast after halfway through Sha’ban , but only for those who are continuing after fasting before halfway through the month. The Shafi’is followed all of these ahadith and said: 
Fasting after 15th Shaban: Permissible?
It is not permissible to fast after halfway through Sha’ban except for those who have a habitual pattern of fasting, or who are continuing after fasting before halfway through the month. 
According to most of the scholars, the prohibition here means that it is haram. 
See al-Majmu’, 6/399-400; Fath al-Bari, 4/129 
Some, such as al-Ruyani, suggested that the prohibition here is to be understood as meaning that it is makruh, not haram. 
Al-Nawawi (may Allaah have mercy on him) said in Riyad al-Salihin (p. 412): 
“Chapter on the prohibition on anticipating Ramadan by fasting after halfway through Sha’ban, except for one who is continuing after fasting before halfway through the month or who has a regular pattern of fasting such as fasting on Mondays and Thursdays”. 
The majority of scholars are of the view that the hadith which forbids fasting after halfway through Sha’ban is da’if (weak), and based on that they said that it is not makruh to fast after halfway through Sha’ban. 
Al-Hafiz said: The majority of scholars said that it is permissible to observe voluntary fasts after halfway through Sha’ban and they regarded the hadith concerning that as da’if. Ahmad and Ibn Ma'in said that it is munkar. (From Fath al-Bari). Among those who classed it as da’if were al-Bayhaqi and al-Tahhawi. 
Ibn Qudamah said in al-Mughni that Imam Ahmad said concerning this hadith: 
It is not sound. We asked ‘Abd al-Rahman ibn Mahdi about it and he did not class it as sahih, and he did not narrate it to me. He used to avoid talking about this hadith. Ahmad said: al-‘Ala is thiqah and none of his ahadith are regarded as munkar apart from this one. 
The al-‘Ala referred to here is al-‘Ala ibn ‘Abd al-Rahman who narrated this hadith from his father from Abu Hurayrah (may Allah be pleased with him). 
Ibn al-Qayyim (may Allah have mercy on him) responded in Tahdhib al-Sunan to those who classed this hadith as da’if and said that this hadith is sahih according to the conditions of Muslim. Even though al-‘Ala is the only one who narrated this hadith, that is not regarded as detrimental to the hadith, because al-‘Ala is thiqah; in his Sahih, Muslim narrated a number of ahadith from him, from his father from Abu Hurayrah (may Allah be pleased with him). Many Sunnahs are narrated from the Prophet (peace and blessings of Allah be upon him) only through one person who is thiqah, but they have been accepted and followed by the ummah. 
Then he said: 
With regard to those who think that there is a contradiction between this hadith and the ahadith which speak of fasting in Sha’ban, there is no contradiction. Those ahadith speak of fasting half of it along with the previous half, and of habitual fasting during the second half of the month, whereas the hadith of al-‘Ala speaks of the prohibition on fasting deliberately only after the month is halfway over, not about fasts that a person observes regularly or that are a continuation after fasting during the first part of the month. 
Shaykh Ibn Baz (may Allah have mercy on him) was asked about the hadith which says that fasting after halfway through Sha’ban is not allowed. He said: 
“This is a sahih hadith as Shaykh Nasir al-Din al-Albani said. What is meant is that it is not allowed to start fasting after halfway through the month. But if a person fasts most or all of the month, then he is following the Sunnah.” (Majmu’ Fatawa al-Shaykh Ibn Baz, 15/385). 
Shaykh Ibn ‘Uthaymin said in his commentary on Riyad al-Salihin (3/394): 
“Even if the hadith is sahih, the prohibition in it does not mean that this is haram, rather it is simply makruh, as some of the scholars have understood it to mean. But whoever has the habit of fasting regularly should fast, even if it is after halfway through Sha’ban.” 
In conclusion, it is not allowed to fast during the second half of Sha’ban, and that is either makruh or haram, except for the one who has the habit of fasting regularly or who is continuing after fasting during the first half of Sha’ban. And Allaah knows best. 
Why is it not allowed to fast after the 15th of Sha’ban?
The reason for this prohibition is that continually fasting may make a person too weak to fast in Ramadan. 
If it is said that if he fasts from the beginning of the month he will become even weaker, the response is that whoever fasts from the beginning of Sha’ban will have gotten used to fasting so it will be less difficult for him to fast. 
Al-Qari said: The prohibition here means that it is disliked, as a mercy to this ummah lest they become too weak to fulfil their duty of fasting during Ramadan in an energetic fashion. But those who fast all of Sha’ban will become used to fasting so it will not be difficult for them. 
And Allah knows best.