B.C.
Give it a try. You'll be sad you missed the wedding especially when you realise how easy babies are to travel with compared to toddlers, I am sure!
I wouldn't worry too much about throwing off his schedule. I have travelled a lot with our now 8-mos-old, and he has been very resilient and flexible.
Ask your hosts if anyone else will be hiring a babysitter on the night, and see if you can either use that sitter or get a recommendation from someone in the area.
In the meantime, you could try babysitting a few times to get some practice. It's probably more for you than for your son -- I'm still not used to the idea myself!
As for freezing breastmilk in advance, can you take a pump with you? You'll probably need only one or two feeds' worth if it's just for the Saturday night.
As someone who is constantly being told to go out, relax a little, and enjoy my life for a few hours without the baby, I must pass on the advice! Have fun.