Scholars differed as to the reasons why the Prophet, sallallaahu ‘alayhi wa sallam, fasted so much in Sha’baan. Their various opinions were as follows:

1-   That he had been unable to fast three days out of every month because he was travelling or for some other reason, so he made up for all of them together in Sha’baan. When the Prophet, sallallaahu ‘alayhi wa sallam, began to do some voluntary acts of worship, he would persist in it, and if he missed it, he would make up for it later.

2-   It was said that his wives used to make up for the days that they missed of Ramadhaan in Sha’baan, so he used to fast because of that. On the other hand, it was reported from ‘Aa’ishah, May Allaah Be Pleased with her, that she used to delay making up for the days that she had missed in Ramadhaan until Sha’baan when the Prophet, sallallaahu ‘alayhi wa sallam, would be making up for the voluntary fasts that he missed, just as he used to make up for the voluntary prayers that he missed. ‘Aa’ishah, May Allaah Be Pleased with her, would do so as she was too busy with the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, to fast in other months.

3-   It was said that it was because this is a month to which people do not pay attention. This is the most correct view, because of the Hadeeth of Usaamah, May Allaah Be Pleased with him,  quoted above, in which the Prophet, sallallaahu ‘alayhi wa sallam, said: “That is a month to which people do not pay attention, between Rajab and Ramadhaan.” [An-Nasaa’i]

4-   Another benefit of fasting in Sha’baan is that it is a kind of training for the fast of Ramadhaan, in case a person finds it difficult to fast when Ramadhaan starts; if he fasts in Sha’baan he will be used to fasting and he will feel strong and energetic when Ramadhaan comes. Sha’baan is like an introduction to Ramadhaan and it has some things in common with Ramadhaan, such as fasting, reciting the Quran and giving in charity. Salamah ibn Suhayl, May Allaah Have mercy upon him, used to say, “The month of Sha’baan is the month of reciters (of the Quran).”

Habeeb ibn Abu Thaabit, May Allaah Have mercy upon him, used to say whenever Sha’baan comes, “This is the month of reciters (of the Quran). By the advent of Sha‘baan, ‘Amr ibn Qays Al-Mulaa’i, May Allaah Have mercy upon him, used to close his store and devote his time to reciting the Quran. He also used to say, “Blessed is the one who reforms himself before the advent of Ramadhaan.”

Ibn Rajab Al-Hanbali, May Allaah Have mercy upon him, said:

Fasting in Sha’baan is better than fasting in the Sacred Months, and the best of voluntary fasts are those that are (observed in the months) closest to Ramadhaan, before or after. The status of these fasts is like that of the confirmed voluntary prayers, which are performed before and after the obligatory prayers and which make up for any shortfall in the number of obligatory prayers. The same applies to fasts observed before and after Ramadhaan. Just as the confirmed voluntary prayers are better than other kinds of voluntary prayers, so fasts observed (in the months) before and after Ramadhaan are better than fasts at other times.

http://300035.dotdothktrade.tech/en/artical/1777