13th, 14th and 15th of the holy months of Rajab, Sha’ban and Ramadhan are known as “Ayyaam-e-Beedh” (the days of surpassing splendor). Imam Ja'far son Mohammad As-Sadeq (peace be upon him) had said: " Whoso prays during the nights of “Ayyam Beedh”, in fact, stands at the door of superabundant favours, bounties and benefits available in these months."
It is highly desirable to observe fast on 13, 14 and 15, have a bath each day, and give alms in the way of Allah. In the night of “Ayyam Beedh” pray a 12 Rak'at Prayer as under:
(i) In the night of the 13th pray 2 Rak'at: In each Rak'at recite Surah Al Fatihah, Surah Al YaaSeen, Surah Al Mulk & Sura Ikhlass.
(ii) In the night of the 14th pray 4 Rak'at: in 2 sets of 2/ 2 Rak'at, each exactly as (i),
(iii) In the night of the 15th pray 6 Rak'at: in 3 sets of 2 of 2 Rak'at, each exactly as (i).
Imam As-Sadeq (peace be upon him): "Whoever performs this prayer in these months his/her sins will be wiped away except the sin of polytheism and he/she will gain all the blessing of these months."
The Holy Prophet Mohammad al Mustafā (Allah's prayers be upon him and upon his holy Household) said: “One who observes two fasts in the month of Rajab nothing in the earth and heavens can imagine the greatness of that person. He would get the reward of a truthful person who has never lied in his whole life-time. He would appear on the Day of Judgment in the similar state. That person would do intercession of others along with other truthful people till they all enter Paradise and settle down with those truthful persons."
“One who fasts for three days in Rajab, Allah (The Almighty) would erect a barrier between him and Hell whose distance would be equal to travel of seventy years. Allah (The Almighty) would say at the time of Iftar, ‘You have right upon Me and My love for you has become obligatory. O angels! I hold you witness that I have forgiven his sins from start till the end"
"One who fasts for sixteen days in Rajab would be among those who are first to ride upon the vehicles of bright light. He would fly from Paradise to the door of Mercy in these vehicles. One who observes seventeen fasts in Rajab, seventy thousand lamps would be illuminated for him at the Sirat Bridge so that he enters Paradise passing the bridge in the light of these lamps. Angels would welcome him saying, ‘Well done’ and ‘Peace be upon you"
There is hadith that states that performing ibadaah on the first night of the month, a Muslim is forgiven all their sins as if they were a new-born and they are given permission to intercede for 70 people who would otherwise have been doomed to torment in the afterlife!
Another hadith states that,
Fasting one day during this month, Allah makes Paradise your right and He will be pleased with you, and He will reward you with a special angel called Ridwan (the angel who is in charge of maintaining Jannat) who will let you enter Jannat.
Fasting two days during this month, Allah doubles all the rewards of the month and He gives you special recognition in His eyes that none can possibly describe.
Fasting three days during this month, Allah will put a distance of 70 years between you and the fires of hell when you cross the sirat.
Fasting four days during this month, Allah showers His mercy on you and protects you from all the evil diseases on this earth.
Fasting five days during this month, Allah showers His mercy on you and protects you from the turmoil and punishments in the grave.
Fasting six days during this month, Allah makes you shine brighter than the moon on the Day of Judgement.
Fasting seven days during this month, Allah shuts the seven gates of hell for you.
Fasting eight days during this month, Allah opens the eight gates of Jannat for you.