Was a guest at a wedding a few weeks ago and took some extra shots purely as a favour with my 12-40mm during the day, and night.
Daytime shots were great when they were composed correctly, however come night time and lack of flash the 12-40 just wasn't fast enough at f2.8, the majority of the evening was on a terrace so lighting wasn't terrible but adequate for a faster lens.
Something like that 25mm f0.95 would be ideal in those circumstances , should be fast enough to help images not be blurry whilst not having the harshness of the flash, in my opinion anyway!
Manual focus could be a bit of a problem though unless you're really quick!