Caravan cannot be played until every player has had their first turn.
The Caravan Caravan is played between two player's turns. Multiple Caravans cannot be played in succession; as soon as one is played you are no longer "between turns". Thus, all other Caravan's must be held in hand until the current Caravan turn ends. Note that the active player (the one who just ended his turn) gets priority to play his or her Caravan before other players.
This event could be argued as being the most used Spellfire card in the game, since it's always beneficial to gain an additional turn. The ability to take an extra turn, and the accompanying chance to build an additional realm, should not be ignored. Since the Caravan is a common card, it shouldn't be a difficult one to obtain.
When using Caravan, it is best played immediately after ending a player's turn, or if drawn as a spoils when defending can be used between other player's turns (in a multiplayer game).
Because the Caravan is a commonly used card, it is considered good etiquette to pause briefly before your turn, to allow an opponent to play the Caravan (the "Caravan Pause"). You must get your hand down to eight or fewer cards before playing The Caravan, since it can be played only after the end of your turn (it is one of the few events that cannot be played at any time).
Being a helpful event, Caravan cannot be negated through the use of Calm (400/CL)