I think you should bring it up about a week before you go, then again a couple days before you go, then the night before. Tell her the truth. Tell her that you are going to a wedding, and you will only be gone for a few sleeps. That she will be with Grandma and Grandpa, and that she can call you on the phone if she needs to. It's not a huge deal. She needs to get used to the idea that you will not always be around, or you may have some trouble on your hands when it's time to go to school
. I think it will do her a world of good to spend time with other people (as it will do a world of good for you to have a some time to yourself, look pretty, and have a great time at the weeding! As long as you're not worrying the whole time that is
). If you make a big deal out of it, then she will think somethings up, and will become upset, and make a big deal out of it. Be as nonchalant about it as you can, making sure though that she knows you love her, and you are coming back soon.
Good luck, and have fun!