How to combat big race anxiety the week/day before race?

Don't know if it helps with the actual feelings of anxiety but.....visualize yourself doing well. It helps to have seen the course, or at least have very good idea of it. Visualize yourself having a good swim, coming out of the water strong, being relaxed on the bike, feeling strong at specific points on the run, winning your AG, heck, winning the whole thing!  I always visualize winning it, even if I don't have a snowball's chance in H..... 

As for controlling pre-race butterflies, I'm probably not the one to ask. I'm the one who's trying not to puke at the pre-race briefing. Just accept it for what it is, a normal part of your body's revving up. Accept that you probably won't sleep well, and that one night's poor sleep probably doesn't affect performance anyway, and that most of the competition isn't getting much sleep either! For me, the only thing that really calms pre-race butterflies is putting my face in the water. So I start the swim "warm-up early! Often it's just floating around but it calms me down and (here in the tropics) keeps me from overheating.

I agree with Hotrunner.  Visualization is a great thing, take time to survey the course, visualize your steps but do not over complicate it by making notes etc, just take a few mental pictures.  Then sit and take time to work through the entire race and visualize yourself doing and feeling great.  Recall the mental pictures from the reconnaissance.

Have a basic race strategy and plan, nutriton, and contingency plans (what am I going to do if...)

Expectations are a killer for an A race.  You build yourself up so much, tell everyone you are training for it, call it an A race, put yourself through an incredible training plan and more, why shouldn't you be anxious? Try underplaying this to everyone except for yourself; after all, we only race for ourselves (charities aside). Underplay it until the very moment you need to kick it into gear; the start line gun.

Expect and accept to get little to no sleep (as all of your previous races), expect the tight transition spot, the crappy swim start position, the horrible parking and logistics of taking a bus to the start line, the fact that you forget your 'foggle' or some other 'non-necessity' that is suddenly now a matter of life and death and... smile.  Take it all in, be happy with the fact that you can start believing that you have a chance to win this race, 75% of people start knowing they are not going to win...

Relax, be mentally prepared, enjoy the day, focus on the now, not the next... You will do GREAT!