«Gathering at the Masjid on the Day of Arafah in the Hanbali School»
Al-Tarif is people in different places outside of Mecca gathering on the day of Arafah after Asr in a masjid, and busying themselves in dua, dhikr and supplicating to Allah Taala, just as the people in Arafah do. (Mughni al-Muhtaj)
«Ruling in the Hanbali School»
Imam Ibn Qudama al-Maqdisi reports in al-Mughni that Imam Ahmad stated that it was permitted to conduct such a gathering on the day of Arafah. This is the relied upon view of the madhab.
Imam al-Mardawi in his work al-Insaf that there was a view attributed to Imam Ahmad that such a gathering was Mustahab, however this is not regarded as the strongest position within the Madhab.
It is also reported from Imam Ahmad that he was asked whether he attended such a gathering, he replied that he did not.
«Evidence»
Imam Ahmad stated (as recorded in al-Mughni) that conducting the Arafah gathering was the practice of a number of the salaf.
Shaykh Taqi al-Din Ibn Taymiyyah in his work Iqtida Sirat al-Mustaqim stated that conducting such gatherings was the practice of companions such as Ibn Abbas and Amr bin Hurayth (Allah be pleased with them) and a group of the people of the Basra and Medina.
Imam al-Nawawi mentions that in the Sunan al-Bayhaqi there is a narration that Hasan al-Basri would conduct such a gathering where he would make dua, make dhikr and people would gather.