You have to eat it the day it's made. Sushi cakes will not keep overnight (the rice will get hard in the fridge and you don't want to eat day old sashimi). Ideally you would pick it up from the shop and eat it within the next hour or two. Or you can put it in the fridge for no more than a few hours (eg. if you pick it up during the day but want to serve it for dinner), and then remove the cake from the refrigerator 15 minutes before serving.