Is this true?
No. This type of question gets posted on Yahoo all the time, but the rumor is absolute nonsense. I worked in the travel industry for 20 years and never saw this happen.
Depending on the route that you're flying, it may be cheaper to fly on certain days of the week or certain times of the year. However, the airlines don't base prices on buying tickets on specific days of the week. (Another rumor that gets posted here a lot is that it's cheaper to buy tickets right after midnight. That's complete rubbish, too.)
Airlines adjust fares all the time based on demand for seats and what their competitors are charging. I've seen fares for one flight change multiple times in a single day. They don't wait for a certain day of the week or certain time of day.