Shimoda Treaty is a 
Russian event regarding 
Japan. It signifies the first official treaty between Russia and Japan
It can happen between 1855 and 1858 if Russia owns Okhotsk (ID 1072), which it does at the start of the game. It also requires that Japan does in fact exist, that it is not a puppet and not in a sphere of influence. Russia and Japan must also be at peace with each other, but the two countries can be involved in other wars.
It has a mean time to happen of 5 months, which decreases by 25% after 1856 and another 5% after 1857.
It improves the relation between Russia and Japan by 50 but has no other effects.