Mustard greens are much tastier hot and fresh the day they are prepared. That being said, you can make them a day or two ahead of serving as long as you keep them refrigerated in an airtight container. You will want to re-heat the greens in a pan with a little bit of oil (choose a flavorless oil like canola or grapeseed). I recommend adding a bit more cider vinegar or a squeeze of lemon juice when re-heating to awaken all those flavors.