There is no dinner served the day you board the ship and you need to have one before you go to the cruise pier. There are some distance between the ship pier and any decent restaurant. The reason there is no dinner is that all the crews are off for a half day after 4-5 days'  downstream or upstream sailing, except some guards and a couple attendants at front desk.

This is the case for most of the cruise ships, except Viking Century Sun and Yangzi Explorer, which dinner is served the day you board the cruise.