The holy Prophet (s.a.w.a.) and the Imams (a.s.) have said:“One of the characteristics of the twelfth Imam, Qaem Aale-Muhammad (a.t.f.s.) is that he would have two occultation: Ghaibat-e-Sughra and Ghaibat-e-Kubra” .
We do not know the real reason behind occultation. However, we can guess that Allah has set the minor occultation for habituating and preparing people for the major occultation. This guess is supported by traditions where Muhaddith Qomi has brought down in the second chapter of“Muntahi-ul-Aamal”  in the discussion concerning biography of Imam Hasan Askari (a.s.). Besides, the tenth and eleventh Imam concealed themselves from the people as much as possible in order to prepare them for the major occultation.
If Hazrat had major concealment right from the beginning, then perhaps Allah’s argument wouldn’t be finalized for the people.
In the minor concealment, a group of people would visit Hazrat; his Imamate would be proved for some; his miracles were made apparent; he would help the people directly and people would recognize him. However, if Hazrat was not in the company of his father for five years and right from the year 260 A.H. (martyrdom of Imam Hasan Askari), the major concealment would have began, it was possible that the argument would not have been finalized for the majority.
Perhaps we can say that by Allah’s Grace and for the sake of finalizing the argument, he spent 69 years of his life in minor concealment i.e., from the year of Imam Askari’s martyrdom in 260 A.H., till the year 329 A.H. In that year, his special envoy, Ali-ibn-Muhammad Samori passed away. By his death, the major occultation began. If we count the minor concealment from the start of Imam Mahdi’s life, it would be 74 years and if we count from the start of his Imamate, it would be 69 years.
The minor concealment is named so because its duration is shorter and while he remained concealed from the common people, yet a few special figures were fortunate enough to visit him. His envoys (four among them being very famous) used to visit him. At times, by acting as an intermediary, others would get chance of meeting Imam. These envoys were reliable figures who would never disclose Imam’s place to anyone. They would take only a very few in the presence of Imam (a.t.f.s.). For this reason, this period was named as minor occultation. From 329 A.H. began the major occultation. Imam (a.t.f.s.) sent a letter (Tauqee) to his envoy and therein declared:
“The absolute (kubra and full) occultation has started” .
During this period, nobody could claim the special envoy-ship or claim to meet Imam by his free will.
The major occultation is longer and Imam is concealed from majority of the people. If anyone is fortunate to see Imam it is by Imam’s own desire and will.
About this matter, one may refer to“Kamaluddin” of Saduq (pp. 434-478) and“Esbaat-ul-Huda” of Shaikh Hur Aameli. In them, they have brought down traditions and narrated incidents of those who have visited Hazrat Mahdi (a.t.f.s.) during the minor concealment or even before; either during Imam Askari’s era of those five years; or after the termination of Imam Askari’s Imamate; either through an intermediary (i.e., the envoys) or without them.