Yokohama is often my base when I'm in Japan for work and I've found that trying to find a room a month or less out is often difficult. I think that, for whatever reason, Yokohama is a popular weekend destination and weekend room scarcity isn't limited to special events or holidays.

I usually stay at one of the Toyoko Inns by the stadium and it looks like those all the Toyoko Inns in the area are full that night, as is the Super Hotel just down the street. You may want to keep checking every day or two - I've had rooms open up occasionally and have been able to grab them.