**UPDATED with ball pics** Opinions? I FINALLY found a gown

Breanna
So I didn't go to prom when I was in high school and my boyfriend knows how much I regret it. A few months after we started dating, he asked me to go to the Marine Corps Ball with him, which is coming up in November at Ceasar's Palace in Las Vegas. FINALLY after months of searching, I have found a dress.
I really love this dress, but my problem is that 1.) I have the worst fashion sense, and 2.) I don't know if this dress is fancy enough, or too fancy for the occasion since I have never been to an event that I have had to wear a gown for. 
I could really use your opinions! 
(This pic is of me trying on the dress at the Tailor's after getting it altered. It's a mermaid style gown with a small tail in the back)