With the sighting of the crescent of Rajab, Rasulullah Sallallahu Alaihi Wasallam would anticipate and begin preparations for the month of Ramadhan. This is two months in advance.
Sayyiduna Anas Ibn Malik (radiyallahu’anhu) reports that Rasulullah Sllallahu Alaihi Wasallam would recite the following supplication when the Month of Rajab would commence:
Allahumma baarik lanaa fee Rajaba wa Sha’bana wa balligh-naa Ramadhan
(Shu’abul-Iman #3534, Ibnu Sunni#660, Mukhtasar Zawaid Bazzar #662, also see Al-Azhkar #549)
Rajab is the second of the “four sacred months” in the Islamic Calendar which are referred to as: “Al-Ash-hurul hurum”
(Surah Tawbah, verse: 36)
The others are : Zhul-Qa’dah, Zhul-Hijjah and Al-Muharram.
Explaining their significance the ‘Ulama mention that during these months, good deeds are more virtuous and evil deeds are more detestable to Almighty Allah. (Lataiful-Ma’arif, pg.163)
As for Sha’ban, authentic Ahadith describe special significance for the 15th night.
Rasulullah Sallallahu Alaihi Wasallam is reported to have said: “Certainly Almighty Allah forgives everyone on this night besides those who ascribe partners to Him and those who harbor enmity for others”.
(Sahih Ibn Hibban #5665, Al-Targheeb,vol.3 pg.459, Majma’uz-zawaid,vol.8 pg.65 & Lataaiful-Ma’arif, pg.194)
Imam ‘Ataa Ibn Yasar (rahimahullah)- a distinguished Tabi’e- said: “After Laylatul Qadr, there is no other night more virtuous than the middle (15th) night of Sha’ban.”
(Ibid, pg. 197)