Sumo tickets?

I am visiting Japan the last week of September. I will be in Tokyo only three full days and want to sight see/eat as much as I can. Is Sumo worth it? I'd love to squeeze it in but I'm thinking twice about it because it takes place during the day (it would be a no brainer as a night out).

Is it possible to keep it as a game time decision or do I absolutely need to buy tickets months in advance?

If it's for one of the first rounds of the tournament, it's definitely possible to buy on the day. Just get up early and be there when the box office opens and you won't have trouble buying the cheap tickets (enough to experience it). The top category fights happen at the end of the day, and you don't need to watch the lower categories if you don't want to. Totally worthy experience as a "once in a lifetime" thing :)

Is Sumo worth it?

Some impatient, uninterested people may not find it worth, but to everybody else I highly recommend it. Especially the 2-3 last hour of the day before the action ends around 6pm.

Is it possible to keep it as a game time decision or do I absolutely need to buy tickets months in advance?

On weekends and public holidays, the studium usually sells out and you should buy tickets in advance. But on regular weekdays, the stadium rarely sells out and you can usually just get tickets easily on the day. Note that September 16 and 23 will be public holidays.

Also, if you have a preferred better type of seat, you should better purchase in advance.

You can buy your Sumo tickets from your own country now, BuySumoTickets.com for the upcoming basho. A very helpful website.

I wouldn't miss the opportunity to attend a sumo tournament, and it doesn't take a lot of time out of the day if you go to the higher ranking games, starting at 4pm and finishing around 6pm.