If your visit to Tokyo is during the period the Grand Sumo Tournament is held (mid January, mid May and mid September) the you should certainly not miss the opportunity to see this in person.
Popularity of the tickets have soared in the last few years, partly fueled by the rise in international visitors. Thus it is becoming extremely difficult to get tickets without paying a hefty charge to a middlemen.
About 300 non-reserved seats (the last row of the arena) do go on sale each day on a first-come basis. Usually you have to show up at the Kokugikan Arena by 7 am, sometimes even earlier. Only one ticket is sold for every person that shows up, in other words one person can't buy four tickets for the family.
When: Normally the tournament starts on the second Sunday and goes on for 15 consecutive days, thus ending again on a Sunday.
What time: The wrestlers in the lower categories start before 9 am, however the top-ranked wrestlers start to appear around 4 pm. This is when a majority of the crowd shows up, though you're free to arrive at any time of the day. The final match featuring the Yokozuna (highest-ranked wrestler) finishes shortly before 6 pm.
During the final 15th day of the tournament, the final match is over around 5:30 pm, with the awards ceremony following that.
How is the view from my seat?
The Kokugikan arena has capacity of around 11,000 with a lower deck and upper deck. There is a clear view from every seat; no pillars that would obstruct the view.
Seats in the lower bowl are the traditional Japanese type where one takes off their shoes and sit on a cushion. There are box seats accommodating 4-6 people. The front row Tamari seats are rarely sold to the public, usually secured well in advance by long-time sponsors.
The upper deck features the usual western-style chairs with decent legroom, and most international visitors will find these seats to be far more comfortable than the traditional ones. (Imagine sitting on the floor with no back support for a few hours - it's quite tough, even for a Japanese!)
For both the lower and upper deck, category A is closer to the ring, category C further away.
Shomen 正面 is the same front angle that you will see on TV broadcasts. Since there is no price difference between the other three sides, these seats tend to sell faster. All of the ceremonial stuff will be done with the wrestlers facing this way.
Muko-jomen 向正面 refers to the opposite side of the ring. One advantage is that it is closer to the Hanamichi, the passageway where the wrestlers appear and exit.
東Higashi/East and 西Nishi/West refers to the seats with a side view of the ring, at least at the beginning of the match. Don't be too disappointed if you get these seats because once the match begins, there's no telling which direction the wrestlers will be going. You might end up having the best angle of what happened at the conclusion of the match.
Other chances to see Sumo
1) Seeing Dohyo Matsuri
The Dohyo Matsuri a Shinto ritual that is done the day before the Grand Sumo Tournament starts.
Open to the public for free, the gates open around 9:45 and the ceremony starts at 10 am. This is a ceremony to purify the ring and wishing for the tournament to be carried out safely without injuries.
There are no matches held this day, but all of the wrestlers in the top category attend this event, so this is a chance to see the wrestlers up close.
2) Going to a Jungyo - regional exhibitions
There are numerous Jungyos, which can be translated either as a regional or exhibition tournament held on April, August and October, a period in between the main tournaments.
While most Jungyos are held in rural communities, a few are held in central Tokyo as well. In the last few years a Jungyo has been held at Aoyama Gakuin University, in the Odaiba district and at the Kitte Shopping Mall near the Tokyo Station, all happening in August.
The program will feature a comical explanation of the rules, chance to see the wrestlers getting their distinctive hairstyle done, and small kids challenging the wrestlers. There are also head to head battles of the top ranked wrestlers towards the end of the program, usually happening around 3 pm. The results in the exhibition don't really mean anything for the wrestlers so the atmosphere is much more relaxed in comparison with the actual tournaments (Imagine a friendly match in soccer).
3) Retirement Ceremony
Wrestlers who played in the top division are entitled to hold a special ceremony at the Kokugikan, usually happening on the weekend after the January/May/September tournament has finished.
The program is similar to that of a Jungyo, and you will get to see exhibition matches of wrestlers in the top division.
The climax is when the top-notch of the wrestler is removed, usually the stable master who has the honor of cutting it off. It's an emotional moment as it truly signals the end of the career for that wrestler.
4) Additional Events hosted by TV Networks
In February, Fuji TV and the national televsion network NHK host a one-day tournament at the Kokugikan Arena.
The Fuji TV event is held on the first or second Sunday while the NHK event is always held on the 11th as a charity event.
Again, the program would be similar to those seen in Jungyos.