An increase in demand causes prices to increase – that basic and intuitive economic relationship appears to fall apart during thanksgiving when supermarkets advertize their discounted turkeys – even though demand must surge then. In 2011 and 2012 Matthew Yglesias explained why this was the case:

• Stores discount the turkey to lure you in…but they jack up the prices on complimentary goods such as stuffing and sweet potatoes.
• Evidence also suggests that instead of giving you standard turkey at lower prices; supermarkets sell lower quality turkey for the discounted price – the idea being that you know that most of the turkey will be consumed by your guests instead of you and so you don’t care about its quality as much.
• The price of higher quality turkeys seems to be going up.

Read more about the economics of thanksgiving Turkey at the source links below.

Source: Slate 2011, Slate 2012