Found out what to edit manually to fix this.
In classes.php, at line 399, it says this:
$where .= " AND WEEK(post_date, 1)='" . $q['w'] . "'";
The “1” indicates Monday, meaning that no matter what settings you have in General Options, it will always show 7 days posts starting on Monday. To fix it, I just replaced the “1” with a “0” (Sunday).
There should be a way to check your settings for “start_of_week” and place that there, but I originally tried the following code and it DID NOT WORK:
$start_of_week = get_settings('start_of_week');
$where .= " AND WEEK(post_date, $start_of_week)='" . $q['w'] . "'";
When using that code, it just defaults to Sunday (which is what I wanted anyway, but if you want a day other than Sunday that code won’t do anything).
Is there anywhere that I can post this as a bug report that WordPress will fix in the next release?