I recommend making it the day that you plan to eat it for best results. The peaches can make the arugula soggy if you make it too far in advance. But if you do wish to make it in advance, combine everything except the peaches up to two days ahead of time. Then add the peaches the day you plan to serve it. You can make the dressing ahead of time as well (and it’s even better after it’s had some time to develop!), just keep it separate.