Let the Purge begin! Haha, no, just kidding.
I'd probably gather enough money and supplies (food, medicine, etc, etc-) to be well and such through it and spend it with family and friends, perhaps doing something exciting and adventurous, after all it'd be the 'last day', so why not spending it with some adrenaline in the system for a few hours and then perhaps travel somewhere with them and look at the beautiful places travelling for some more hours and after that simply relax and wait for the world to be over, eventually.