The Grand Ayatollah Shobairi Zanjani answered a question about Ziarat Arbaeen.
Question: Considering the large crowd gathering for Ziarat Arbaeen; is Ziarat Arbaeen special to the Day of Arbaeen or is it included in days before and after Arbaeen?
The Grand Ayatollah Shobairi: The special reward (Thawab) of Ziarat Arbaeen is for someone who performs Ziarat of Imam Hussain (AS) on that day. But those who in these particular times succeed to perform Ziarat, will be granted the reward for honouring God’s rites, and to avoid large crowd, perform Ziarat after or before this day, God willing will be granted the same reward of Ziarat Arbaeen.
source : shafaqna