Ijtema

As salaamu alaikum wa rahamatullahi wa barakatu Muftisaab. I am a bit confused over the issue of Ijtema please advise on the following
1) Is there any reference of holding of Ijtema by huzaoor sallalau alaihiwa sallam or the sahaabas??
2) When did this practice originate and what was the purpose, is there and deeni benifit to attend it.
3) Is the attending of at least the DUA on the last day is of great im;importance and benefit.

I am being pushed by my friends to accompany them as its a huge Ijtema with almost 2.5/3 million people attending, they say there will be a lot of barkat and Allahtala accepts all duas in such gatherings.

Awaiting your response.

Jazakallah khair