If you're asking about the one directly below the lens with a light arrow pointing right and a dark arrow pointing left, then you're referring to the lighten/darken control.
If images seem too dark, slide the control to the right, and if images are washed out or too bright slide the control to the left. It's very much a trial and error control in that you judge what's needed by the next photo you take.
One other point: the pictures you intend to take will be irreplaceable, so it's vital for you to understand that remaining stocks of original Poloaroid film are all so long out of date now that the built-in batteries are either totally dead or will allow maybe one or two shots before dying. When the battery dies, the photos cannot be used, and Polaroid packs have never been cheap. Old stocks may also have stale photo chemicals so pictures may not develop properly or may have odd colour casts. Refrigerated photo packs might have good chemicals but you only have the vendor's word that they were kept cold and they will have flat batteries as cold kills batteries. Vendors will also not usually guarantee the material to be usable, and when they do the guarantee is just for the cost of the material, not the lost images. You can use photo packs with flat batteries but need to adapt the camera to accept a stable external 4.5v dc power supply, such as an external battery. Even with an adapted camera, you will always be taking pot-luck on the results using original Polaroid photo packs.
Alternatively, you can get new photo packs via The Impossible Project
. It's vital to realise though that it's NOT the same as the original material. The dyes are different for starters, and you only get 8 shots per pack rather than 10. It's also nowhere near as robust or fade-resistant, so once you get your images it's essential to scan them if you wish to preserve them as the originals will rapidly fade. The website for the product will tell you far more as will the user forum. It's really a specialist medium which is more suited to artists than for archival memory preservation such as you intend.
Good luck with the wedding, I hope that you get to have fun with your camera, but make sure to run a photo pack through it before the big day to check it's fully functional and also ensure that any important photos are also taken with a conventional film or digital camera.
I hope that my reply has been of use, if so then please spare a moment to rate my answer.