Don't know if you are going to TDL or TDS on 9/27, but both websites predict TDL is fairly crowded on 9/27. 9/28 seems to be less crowded if you are flexible with dates. TDS seems to be not that crowded on 9/27 (but again, 9/28 is a better day).
My week runs October 11 through 16, and Monday the 11th is a national holiday.
I read somewhere that a lot of Japanese will stay on a day after the Bank Holiday. So the Tuesday could be a bit crowded too. Although Halloween can get very crowded no matter when you visit. I'm expecting decent sized crowds when I visit between the 13th to 17th of September. I think DisneySea has the potential to be more crowded than Tokyo Disneyland this year. Hopely with both parks offering something new for Halloween, it will spread the crowds out more evenly. For me I'm hoping to catch all the Halloween entertainment and BraviSEAmo for the final time.