Thanks to all for the good advice. Here's the result:
Made the Guacamole at about noon for an 8:00 PM party. My wife's family who are Mexican American are staunch believers in the put the pit in to keep it from turning brown, and I have always done that. So did it again (I mashed about 14-16 Avocados and put in 3 of the pits) I have also always put a lot of fresh lime juice in my gaucamole and did that, including loading up on the top of the guacamole. I also added a very small amount of the Fruit Fresh, which someone here on CH had recommended. It is basically powdered Vitamin C used for canning which prevents oxidation. Finally, I covered the guacamole with plastic wrap and made sure to try and get out all of the air. I then put on a second layer (I noticed that a poster later recommeded doing it; I did it just because I thought 2 is better than 1)Result: Brought out the gaucamole at 8 and it was as green as when I put it in the fridge and tasted just as fresh!