In my case, my host asked me if I could arrive a few weeks early. Additionally, I wanted to visit a few friends before starting work. The only thing that they told me at orientation was that I had to get an invitation letter for the Visa from my host, since EAPSI wouldn't be responsible for me before the program starts. So I told my host when I planned to arrive and when I planned to start working. This might be a bit different for Japan. Still, I don't think most hosts will mind you telling them that you will arrive earlier to do some tourism or to visit friends.