Average cruise fares vary considerably by itinerary and season. Published rates are highest for the most unique and desirable itineraries, as well as during peak summer months, when most North Americans plan for vacations in Europe. Europeans are known to prefer late-summer holidays, with August being the busiest month. Typical daily per diems on a luxury line such as Silversea or Seabourn can be as much as three times the cost of a cruise on a mainstream line such as Royal Caribbean or even a premium Celebrity Cruises ship. It goes without saying that longer cruises are naturally more expensive.