Imam Hasan al-Askari

Hasan ibn Ali ibn Muhammad (c. 846 – 874) was the 11th Imam of Twelver Shia Islam, after his father Ali al-Hadi. He was also called Abu Muhammad and Ibn al-Ridha. Because Samarra, the city where he lived, was a garrison town, he is generally known as al-'Askari ('Askar is the word for military in Arabic).

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Sayyed Mohammad Al-Musawi, Sayyed Mohammad al-Musawi is originally from Iraq and heads up the World Ahlul Bayt Islamic League in London. Other than being involved in various humanitarian projects, he frequently responds to... Answered 5 months ago

As the Abbasid government rulers' pressure on Imam Hasan Al-Askari (AS) and his family was tremendous, the disclosure of Imamate of Imam Mahdi (AS) was among the near circle of the most trusted persons. Imam Hasan Al-Askari (AS) told very few most trusted believers and allowed them to see his successor Imam Al-Mahdi (AS). These are names of some of those who saw Imam Al-Mahdi (AS):

1. Othman ibn Sa'eed Al-Amri who became later the first Safeer of the Imam Al-Mahdi (AS).

2. Muhammad ibn Othman ibn Sa'eed Al-Amri, who became later the second Safeer of the Imam.

3. Al-Husain ibn Rooh.

4.  Ali ibn Muhammad Al-Samari.

5. Lady Hakimah daughter of Imam Al-Jawad (AS).

6. Ibraheem ibn Mahziyar Al-Ahwaazi.

7. Ahmad ibn Eshaq ibn Sa'd Al-Ash'ari.

8. Ahmad ibn Muhammad ibn Al-Mutahhar.

9. Ismael ibn Ali Al- Nawbakhti.

10. Ali ibn Ibraheem ibn Mahziyaar Al-Ahwaazi.

11.Kaamil ibn Ibrahim Al-Madani.

12.Allaan Al-Kulayni.

13. Muhammad ibn Ismael ibn Musa Al-Kadhim.

14. Al-Hasan ibn Wajnaa' Al-Naseebi.

15.Ibrahim ibn Idrees.

16. Ibrahim ibn A'bdah Al-kNisabori.

17.Ibrahim ibn Muhammad Al-Tabreezi.

18. Sa'ad ibn Abdullah ibn Abi Khalaf Al-Ash'ari.

19. Ali ibn Muhammad Al-Shimshaati.

20- Muhammad ibn Al-Qasim Al-Aqeeli. 

There are hundreds of well known pious believers who saw Imam Al-Mahdi (AS). Whole books were been compiled on this subject by big scholars.

Wassalam.

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Amina Inloes, Amina Inloes is originally from the US and has a PhD in Islamic Studies from the University of Exeter on Shi'a hadith. She is the program leader for the MA Islamic Studies program at the... Answer updated 3 years ago

I am not personally aware of narrations specifically encouraging the visiting of those sites. (Someone may correct me!) However, there are ziyarat texts narrated for them. 

However, there are narrations encouraging the visiting of all of the Imams, which would include them, and it is good to go and pay respects as a gesture of loyalty and to help maintain a tie with the history and people who once walked there and were laid to rest there. Some people might also want to go as a form of resistance to the oppression that Shi'a face today and to make a stand to show that we will not be afraid to go, and that is also good. 

That being said, there are a lot of narrations emphasizing visiting other places, such as Mecca, Medina, Karbala, Najaf, and Mashhad, so I would personally place the emphasis on visiting those places first.