Yes. Make the jelly and refrigerated for up to 3 days ahead, but DO NOT unmold or cut the jelly until close to serving time. When agar jelly is uncontained, water will slowly seep out because it's an unstable gel. A small amount of water loss will not affect how it tastes, so if it has to sit for a couple of hours unmolded, that's totally fine. But you want to avoid it when possible; so it's best to cut or unmold as close to serving time as possible.