Hi tiger and welcome.
I also went on a mission to lose weight for a wedding...my oldest son's wedding...at the end of August of this year. I started around the beginning of this past summer and have lost almost 55 lbs. since then and am down to around size 10-12 now, I'd say. I wear men's jeans and my 32" waist ones are now getting loose and I've not yet tried on any women's dresses, etc. to check my exact size now....but I was up to 40" waist jeans when I started and buying tops that were 2X, whatever size that translates to (not totally sure). I can fit into med/lg. tops now but I usually buy ex-lg. just because I don't like my tops tight and have a rather big bust (D)....though have gone from a 40D to a 36D now.
Now I can enjoy visions of all the great outfits I'm gonna be able to try on as mother of the groom......and it's great.
I had a good bit more weight to lose than you do and was able lose 55 lbs. of it in about 7 months....not to mention that I'm almost 58 so I'm sure your metabolism is a heck of a lot better than mine is at your young age. So you should have NO problem doing this by December....probably a good bit earlier, in fact.
South Beach falls in the category of a low glycemic index plan, right? I've lost all my weight on Atkins. I love low-carbing...it's the perfect diet for me. And the only exercise I've done is walking. I think you'll have great success.
deena