..."the largest gathering of brides, in wedding dresses"...and then a pub crawl afterwards...This is in Sept, starts at 4pm....goes through evening...as the evening goes on....what do do about lights? and with me on foot for quite a bit of distance...I can't carry a ton of equipment by myself, and I'm likely to be a one man band here. I have my manfroto monopod with fluid head...I'll be bringing that...I have my sling bag...and figure I can carry my 70-200mm f/2.8.....my 17-40mm f/4....But I have no portable lights. Should I invest in some sort of small, portable LED lights?
I got a wedding reception at a dark venue with a 5D3, Manfrotto tripod, 17-40, 24-105 and 70-200 2.8 IS II. The material looked good even at ISO 12,800. Yes there was some noise but it's so dark you can't get the footage any other way and it looks atmospheric. Mic was Rode VideoMic Pro, but audio wasn't vital since much of the final material was covered by music added in post.
If you need to capture spoken word audio, using a remote mic or external recorder is good, if possible.
I used manual focus and a Zacuto EVF Pro, which was very useful. The 70-200 2.8 is a superb video lens even in crowded venues since it allows highly selective focus on the subject. It makes everything look cinematic.
I have LED lights but did not consider those since it can be intrusive if you want candid material. For more formal shots they're very helpful.
IMO using manual focus and a focus aid such as a loupe or EVF are vital for this type of event.
As others said, you really need another camera or assistant. Changing lenses all the time is a good way to miss key happenings.
If you have no recharging opportunities, you'll need at least three batteries, preferably four or five. It's better to not even try and recharge -- it's just one more drain on your limited bandwidth.
Whatever your configuration, you'll need practice before the event to make sure it all works. It is extremely easy to forget to turn on an external mic, forget how to change ISO while rolling video, forget how to adjust mic gain when the quick control dial switches to touch mode, etc.
For a loupe or EVF, all the fittings, screws, cables, etc. should be thoroughly checked out and practiced beforehand. If an EVF or field monitor is used, they have their own menus, modes and controls which should be well practiced.
Remember the 70-200 requires a tripod adapter (which comes with the lens); don't forget to take that. Have several quick-disconnect plates so you don't have to change from camera base to lens tripod adapter.
If it's the Manfrotto 561 BHDV monopod, test the ball mount thoroughly and make sure it doesn't stick or pop. Some of them do and can easily spoil a take by imparting a jerk to an otherwise-smooth pan or tilt motion. If it sticks you may be able to lubricate it, but do all that beforehand.