Why is the first day of the week named for the sun?
The answer is fairly ancient. Scholars believe the astronomical naming of days goes back to the Neo-Babylonian empire, dating to around the time of the Jewish exile in Babylon, though the first written record of the system is from the Roman empire era.
In this naming system, each day is broken up into seven hours, and each hour given the name of a prominent god after whom a celestial body was named. The first hour of each day gave that day its name.
Sunday, therefore, is named after the sun because that day of the week began with the hour of the sun.
This naming system is quite extensive throughout the world, and it came to English via the Germanic language. It’s interesting to note, however, that other European languages without Germanic influence have instead chosen a name for this first day of the week that means “the Lord’s Day.”
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Roseanna M. White is a bestselling, Christy Award winning author who has long claimed that words are the air she breathes. Having successfully launched two homeschool grads, she now spends her time writing fiction, designing book covers, and pretending her house will clean itself. Roseanna is the author of a slew of historical novels that span several continents and thousands of years, as well as a fantasy series and contemporary mysteries and romances. Spies and war and mayhem always seem to find their way into her books…to offset her real life, which is blessedly ordinary. 
What is that alternate name for Sunday?
It’s “sun-day” in Chinese and Japanese too, but I don’t know the origin in those languages. Interestingly, in Chinese, the rest of the days of the week are just numbered 1-6, but in Japanese they’re named for other natural things (moon, fire, water, wood, metal, earth).