largest religious gathering which is held every year

https://www.google.com/search?q=Karbala

Over 19 million people from 40 countries around world participate this pilgrimage, making it second largest gathering in world. Even though Kumbh Mela is larger in population, it is only held every three years, 

https://freeglobaluniversity.blogspot.com/search/label/Kumb

and hence the Arbaeen pilgrimage is the largest gathering held annually.Some Sunnis, Christians, Yazidis and people of other faiths also participate the festival

https://www.google.com/search?q=Arbaeen

Org
https://whoishussain.org/

Site
http://www.islamicgathering.com/

Pilgrimage
https://en.wikipedia.org/wiki/Arba%27een_Pilgrimage

http://www.timeanddate.com/holidays/iran/arbaeen

Month

https://en.wikipedia.org/wiki/Muharram

http://www.timeanddate.com/holidays/us/muharram-new-year

Tube

https://www.youtube.com/results?search_query=arbaeen

Slides
http://www.slideshare.net/search/slideshow?searchfrom=header&q=arbaeen