Stocks go up and down all the time. There are a lot of theories out there that stocks go up or down at certain times. For instance stocks are supposed to go up in January.
These are all really superstitions and if they are true at all it is because enough people believe in them. You see stocks go up and down because of supply and demand, if more people want to buy it then sell it, it goes up. If more people want to sell it then buy it then the price of the stock will go down.
If enough people believe that stocks go down on X date and act on it, that will probably have some impact on what happens. Of course I still wouldn’t put my money on a superstition, but it can be fun to keep in mind.
