When this event goes on sale, and to whom before that. presales is the named windows that open part of the house early, oldest first, each with a start and an end; an end of null means the window runs to the event. onsale is the public window, null when the event states neither bound. Most events state no presale at all, so an empty array is the normal answer and does not mean a presale did not happen. Costs one unit.