The Romans called the second day of the week dies lunae, or day of the moon. Modern languages that come from Latin have similar names- lundi; in French, and lunes in Spanish. But the groups that spoke Germanic languages substituted their own word for the moon and came up with Monandag, which developed into the modern English Monday.
In ancient times, Monday was considered an unlucky day. This
may have been because there were many superstitions about the moon. Some people
even thought that gazing at the moon could drive a person insane. (In fact, the
word "lunacy" comes from the Latin word for moon.) Monday was unpopular
for another reason in days gone by. It was washday - and washing all a family's
clothes by hand was a lot of work.
Even today, Monday is an unpopular day. This has less to do
with superstition than with the fact that Monday marks the end of the weekend. After
two days off, most people have to wake up early and head back to school or work.
But some Mondays are better - in the United States and a number of other countries,
many holidays fall on Mondays. And that gives everyone a three-day weekend. (Grolier New Book of Knowledge Encyclopedia)