My recommendation is that you send a save the date announcement of some sort at least one year in advance. Depending on the location and nature of your reunion, such as a family reunion cruise, for example, a two year notice would be even better. The farther away an event is or the more it costs, the more advance notice you should give. Of course you'd have to send reminders and updates along the way but I personally think it's always better to error on the side of too soon when it comes to reserving times and spaces.
