I am also a natural brunette and happily so for the first 30 years of my life, until I spent the next year a highlighted blonde. I also decided to go chocolately brown due to fear all my hair would turn to straw/fall out if I continued being blonde (and also I think dark hair of an evening out looks more dramatic) so I asked my hairdresser to dye it brown.
Bear in mind a professional did this-I was horrified as the bleach meant my hair was extra porous and it absorbed the dye making it extra dark and very matt and dead-looking! I also looked washed out as obviously your make up changes when you go lighter.
If you really must dye it yourself, choose a shade a few shades LIGHTER than what you want it to be, as it will go darker anyway due to porousness of hair. That way you won't go through the trauma I did of not wanting to go out in public!!!!
Good luck and maybe wait until New Years Day or you may end up spoiling your New Years Eve if you don't like the results! (but hey-it may turn out ok-I hope it does fingers crossed).