There are strong reasons in various narrations (Hadiths) which indicate that the appearance of Imam Mahdi (AJ) is a sudden event. Some of these narrations are explained below.

1.Divine Command for the appearance will happen (suddenly) overnight. Great events require a lot of arrangements to be made; but in this the greatest of the events and establishment of Divine Justice, Allah (SWT) will display Divine Power and will prepare all arrangements overnight. It is narrated from Imam Ali (AS) who narrated from the Prophet of Islam (PBUH) who said: Mahdi (AJ) is from us Ahlul Bait (AS) whose case of appearance will happen overnight [1].
2.Undoubtedly, sudden and overnight preparation is not compatible with setting a time. The appearance of Imam Mahdi (AJ) according to the Prophet (PBUH) is likened to the Day of Judgement as is mentioned in Ayah 187 of Surah Al-A’araf: “All too suddenly it will come upon you.”
3.One of the comparisons used by infallibles (AS) for the appearance of Imam Mahdi (AJ) is to liken it to a bright meteorite. It is clear that the appearance of meteorite is sudden and unpredictable. It is narrated that, after explaining about the occultation of Imam Mahdi (AJ), Imam Baqir (AS) said: Then, like a fiery meteorite [2]. The same words have been mentioned by the Prophet of Islam (PBUH) [3].

[1] Kamalu-Deen, Vol. 1, Page 152.

[2] As Above, Page 324.

[3] As Above, Page 27.

Source: en.shafaqna.com