We are in the USA, and like many other countries our day starts with Sunday (not Monday.)

I can't tell you how many times we accidentally schedule emails to go out on the wrong day of the week because we're hardwired to assume the "Schedule At" field's date picker UI starts with Sunday as we're accustomed to.

I'm not saying we want to force every other user to accept a Sunday start of the week, but it sure would be nice to have an option for us and the 66 other countries that start their weeks on a Sunday.