Trade between merchants and caravans is always best done at a town or way point where it's safe to unpack and display your goods. Tiger's Crossing has been around long enough that caravan routes have become established, and caravans are starting to arrive on certain days once every two months. Caravans will come and go on a relatively random schedule, but for a short period around a weekend every two months, all the caravans will pass through at roughly the same time, thus allowing them to trade among one another on their way to other destinations. When that happens, work increases, jobs are more available for mercenaries, and everyone gets a little more excited.
That day is referred to as "MARKET DAY" and is always the day of the event.
