As always, Cecil is here to help and maintains a calm, amiable demeanor even if Mina needs to yell or huff or complain about him. He's known her for a long time and he knows that the best thing he can do for her and for himself is to take all of it in stride and try to help her get done what needs to get done. He'll haul boxes, pin backdrops (unless that involves getting on a ladder, sorry!), blow up balloons, and all the rest. Just tell him what you need, Mina!
...Admittedly, the rush to get all of this done makes the whole afternoon exhausting and he's glad when it's all finally done and he can head off to get ready for the masquerade. Before he leaves, though, he tells Mina that everything turned out really lovely and thanks her for her hard work. Because, despite anything else, the girl does put her heart into this.