Why do Americans have Sunday, as the first day of week, on calendar ?

Many other countries start with Monday, in accordance with international standards (ISO 8601). Our own practice dates back to the Romans naming days after what they called the seven planets, starting with the sun (Sunday) and the moon (Monday), and ending with Saturn (Saturday). (The other names have since changed.) The day of rest is generally Friday for Muslims, Saturday for Jews and Sunday for Christians, but the Bible makes the Sabbath (Saturday) the last day of the week. Hence the effort to get the world in sync.

For the same reason that it appears thus on some calendars in Britain.