Fasting the day of doubt is haram because the Prophet Muhammad (peace and blessings of Allah be upon him) forbade the Companions to fast it

Fasting in Sha’ban in the second half is not allowed 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. For more, see the detailed answer.

There is nothing wrong with making up a missed Ramadan fast in the last half of Sha’ban.

There is no sahih hadith concerning the virtue of the night of the fifteenth of Sha’ban. With regard to Allah’s descending on the 15th night of Sha’ban, there are hadith which are weak, but the reports that He descends every night are proven and sahih, so the night of the fifteenth of Sha’ban is included in that. For more, please see the detailed answer.

It is not prescribed to spend laylat a-nisf min Sha’ban in prayer or to fast on this day, or to single it out for certain acts of worship.

It is recommended to fast a great deal in the month of Sha’ban. The Prophet Muhammad (peace and blessings be upon him) used to fast the whole of Sha’ban. For more, please see the detailed answer.

There is no authentic hadith about the virtue of Rajab. The hadith which reads: “Allhumma barik lana fi Rajab wa Sha’ban wa ballighna Ramadan” is weak. However, there is nothing wrong if a Muslim asks Allah to let him reach Ramadan. For more, please see the detailed answer.

What is narrated about praying, fasting and worshipping on the 15th of Sha’ban is fabricated. So it is not permissible to follow such hadiths or to act upon them, whether that is in doing righteous deeds or otherwise. For more, please see the detailed answer.