If it is Sunday just East of the International Dateline, what day is it just west of the International Dateline? I think it's Saturday, please verify.
correct.http://aa.usno.navy.mil/faq/docs/international_date.html
Yes, you are correct. Just west of the International Dateline, it would be Saturday. The International Dateline is an imaginary line that roughly follows the 180° longitude line, and it marks the change in calendar days. Moving from east to west, when you cross the International Dateline, you go forward one day. Therefore, if it is Sunday just east of the International Dateline, it would be Saturday just west of it.
To verify this, you can visit the website you provided, which is the U.S. Naval Observatory's website. The page you linked to explains the concept of the International Dateline and how it affects the calendar date.