Just tell the truth and apologize. In my experience guys are known for not remembering these things anyway, so it's probably not as big a deal as you think to her.
And go out for a special night, it's not too late. Better late than never.
good luck.
I thought you had broken u with ur girl and were getting deported
If this is a 9 month anniversary or some gay shit like that than hand over yer man card or tell her it's not a real anniversary and she should STFU