you could set up an event craft shop where once they have received "event tickets" they can go use them to craft an item like mystic dress.
and example under a shop NPC would be:
/* Event Bob */
[npc5]
{
[craft]
Item = 133 (we'll say this is mystic dress)
[ingredient] Item=50 (Event Ticket) Ammount=5
}
that would be the simplest way, then all you have to do is give out event tickets or find a way for players to obtain them.