Summer is not the best season to visit Japan except Hokkaido because of climate. It is very hot and humid. But local demand is quite high. Especially Obon season is one of three busiest period in Japan. Obon period is usually from August 13 to 16. In this year 2014, August 17 is Sunday. So the last day of Obon period is 17 of August. The trains to major cities will be very crowded. On the other hand, trains from major cities will be very crowded on August 13. Some of Japanese will start Obon vacation on August 10, Saturday. Anyway please keep that mid August will be very busy in your mind. You must reserve a seat if you travel in this period. Otherwise you have to stand up on the isle or passengers deck in very crowded train.
Both Cassiopeia and Hokutosei very popular. Many tourists have a plan to take one of these trains. However these overnight trains cannot be reserved online. The only way to reserve it from outside of Japan is calling Odekake net 078-341-7903 in Japanese and you have to pick up your ticket in JR West, Shikoku or Kyushu stations. You cannot pick up it in other areas, like Tokyo. Otherwise You need to be the reservation window at the station in Japan. Ticket is started to sell one month prior to the departure date. That means it may be sold out when you arrive in Japan. So you must have a back up plan when you try to make a reservation after arrival.