Does anyone know why (or if) flights are cheaper on a Tuesday night?

I have a friend who searches for lots of flights and she told me today that cheap flights are usually found on Tuesday nights.

I guess the reasoning behind this is that the prices are set by the major airlines on Wednesday. 

I have no idea if this is like gas prices where Tuesday night is like the last night before a price change so you need to know if prices are going up or down.  If airfares are getting cheaper, then it would be better to wait for the new prices on Wednesday.  If flights are getting more expensive, then it would be best to buy a flight on a Tuesday night. 

Again, I have no idea if this is how it works or if Tuesday nights just have the lowest prices for some odd reason.  Since getting the recommendation to purchase flights on a Tuesday night, I haven't had enough Tuesdays to accurately check this hypothesis.

If anyone has information about this Tuesday night theory, I would love to hear more about it.