Definition

A student in my Sunday school class recently asked why Sunday (the day of the week) is named Sunday. I guess this is a good question for each day of the week.
What is the origin of the names and/or definition.
Thanks in advance for your help

If you can refer me to web sites that will help this type of research I would appreciate the information for future use.

I found this via 'Ask Jeeves' - www.ask.com (or www.ask.co.uk), which is quite good at finding answers to questions in proper English - it's where I found this site for that matter.