You got a couple replies related to the mechanics of tax selling (pertinent dates, wash sale rule, etc.) but your original question asked about the CONCEPT behind the whole thing.
The driver here is the way income tax works on stock sales. The first is taxation. In the US, any capital gains you have on stocks you sell are subject to income tax. However, any capital gains offset by losses are not taxed, so there is an incentive for those with gains from other stocks during the year to sell some of their losers late in the year to offset some of their gains and save on taxes. Additionally, up to $3000 in losses above the amount of gains you have can be used to offset other income and shield it from taxation. Naturally this tax selling tends to happen late in the calendar year.
This is an example of where the CALENDAR introduces a factor influencing the price of certain stocks that is not in any way related to the fundamentals of those companies, and can provide a buy opportunity in such stocks.
If I could afford to buy all of them, I would not need to buy any of them and I sure wouldn't be spending time on the message boards!
