In the last couple events, I was able to go into the event one last time after it ended to use the silver.
I believe I was in the screen with the event girls and monsters when the events ended. When it ended I got the silver from the ranking reward, then it kicked me back to the main screen where you can choose the different modes. The event pic still showed the event girl but it was grey out. When I clicked on it I was able to go into the screen with the event girls and monsters one last time and accessed the event chest. When I left this screen again the picture in the event mode would show the boat with the caption "coming soon".