Halloween in Tokyo

I'll be arriving in Tokyo on the morning of 31 October 2016 and intend to head down to Shibuya, Harajuku and Omotesando immediately after I leave my luggage at the hotel. 31 October is Halloween and I understand people gather in droves at Shibuya in costumes to celebrate the occasion. I've seen on youtube the massive crowds and heard over news networks Tokyo police intends to deploy extra manpower as they were unable to cope with the crowd causing major traffic disruption last year. I would like to be able to experience some of this excitement but as I'm with a few friends who will be first time visitors to Tokyo, I do not want to lose them in the crowd (I've been to Japan several times before and know my way around). My questions are:
1. Since 31 October is a Monday and may not be a public holiday in Japan, will the crowds still gather on that day or will all the celebrations be on Sunday the day before?
2. If the crowds do gather on 31 October, what time will the main bulk of them start to gather and where will the numbers be greatest?
3. Will the crowd spill over to Harajuku and Omotesando?
Would appreciate some advice regarding the size of crowd expected and what/where to avoid so as to help me enjoy the atmosphere without having to worry about being separated from my friends and leave them stranded in a strange new place.

Still hoping for a reply for my post.

ANSWER:
1.As you say, 31 October is a public holiday in Japan, So some events will be held on the 30th or 31th of October. (For example:
in Roppongi,Ikebukuro,Harajuku,Omotesando)
Halloween parade in Harajuku and Omotesand will be held at 11:00-18:00 on the 30th.
2. I think after 5 pm, So the majority of people finish work at about 5 pm.
Sorry, I don' t know Most gathering place.
3. Maybe on 30th.
I recommend that you go to a restaurant in downtown.

Sorry for my poor English.

So am I to understand that Monday 31 October 2016 is not a public holiday for Halloween in Tokyo and Shibuya will be like any normal weekday with the crowds only starting to gather for the celebrations after 5.00 p.m. when people finish work?
On that day I'll be at the Shibuya/Harajuku/Omotesando area between 10.00 a.m. to 4.00 p.m. and hope to see some of the people dressed in their Halloween costumes along the streets.
Anyway Benat21, thanks for the info I have gathered from your reply to my post.