The best way to get to Japan-Tag is by public transport. From Düsseldorf central station, you can either get to the venue on foot, with a walk through Little Tokyo or by public transport. The subway lines U75, U76, U77, U78 and U79 will take you to Heinrich-Heine-Allee, from there it is only a few minutes walk to Burgplatz. Alternatively, you can take the streetcar lines 706, 708, 709 to the Landtag/Kniebrücke stop.
The best way to check the connections is via the Rheinbahn app.