What is the religious significance of the three sacred months and what is the ruling on fasting during these months?

The months of Rajab, Sha'ban, and Ramadan, commonly known as the "three sacred months" among the public, are considered blessed months. Indeed, when the month of Rajab began, the Prophet Muhammad (peace be upon him) would supplicate, “O Allah! Bless us in Rajab and Sha'ban and allow us to reach Ramadan.” (Tabarani, al-Mu‘jam al-Awsat, 4/189 [3939]; Bayhaqi, Shu'ab al-Iman, 5/348-349 [3534]) Fasting in the month of Ramadan is obligatory. (al-Baqara, 2/184-185) As for the months of Rajab and Sha'ban, hadith sources report that the Prophet (peace be upon him) would fast more voluntary fasts in these months compared to others, but he never fasted an entire month outside of Ramadan. (Bukhari, Sawm, 52-53 [1969-1973]; Muslim, Siyam, 173-179 [1156-1157]; Abu Dawud, Sawm, 54 [2428]; Ibn Majah, Siyam, 43 [1741]) Therefore, there is no religious basis for fasting continuously throughout Rajab and Sha'ban. A person may fast as many voluntary fasts as they wish in these months, provided they are healthy and able to do so.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu