The history of the names of the days of the week is a tangled one. The Greeks named the days of the week after their gods, but when the Romans were supreme, they substituted the names of their favorite gods for the original Greek names. However, with English being a Germanic language, it’s perhaps no surprise that our current week has several days named after Germanic gods.
Most people who advise on writing tell you to check the spelling and grammar on your work. This is good advice. However, the trouble is that the tools we often use just aren’t up to the job. The MS Word grammar checker leaves a lot to be desired.