The event will run until November 15 (inclusively). You can summon your harbingers anytime after that (even in a year from now!). Just make sure you turn in all your red crystals because Tomas isn't likely to maintain Halloween stocks after that. He'll be too busy preparing for the Holiday season!
Why do you make items that are only useful for Halloween and not items which retain some sort of usefulness other than deco after the event is over?
Think about it this way: Why would you want to design junk into an in-game system?
If these red crystals were resellable for a small amount of gold to gem shop NPCs and could be purchased in small quantities they would maintain their use as house deco for those who want them, and at the same time not become another junk item found at IDOCs everywhere.
Part of your job as a game designer is to think 'What impact would thousads of this item have six, twelve, twenty-four months etc. down the track for our game server backups?' I don't think tossing people items which will only have a single use would do UO any good in the long term.
Please, even if it is after the introductory phase of this item, redesign it so that it is more compatible with the UO system as a whole, and doesn't become another item commonly left to rot at IDOCs.
Cheers ...
DarkVoid