Charing Cross (Bakerloo and Northern lines) is the closest Tube station, with an entrance/exit on Trafalgar Square.
The following stations are within a few minutes' walk:
Leicester Square (Northern and Piccadilly lines)
Piccadilly Circus (Bakerloo and Piccadilly lines)
Embankment (Bakerloo, Northern, District and Circle lines)
Charing Cross mainline rail station is about a three minute walk from the Square.
The Japan Matsuri Returns to Trafalgar Square.
The free event has lots of stalls and shows and other entertainment along with street food for fans of Japan.