Is voluntary fasting in Rajab and Sha'ban superior to other months, and is it sunnah to fast on the 27th of Rajab or the 15th of Sha'ban?

Rajab is one of the sacred months in which fasting is generally encouraged, but no specific hadith has been authenticated regarding the virtue of fasting in it alone. As for Sha'ban, the Prophet, peace and blessings be upon him, used to fast frequently in it. Singling out a specific day in Rajab for fasting, such as the twenty-seventh, or celebrating the night of Al-Isra wal-Mi'raj, is an innovation (bid'ah), as is singling out the fifteenth day of Sha'ban for fasting.