Prophecies and it's lack thereof

Was amused by the self-fulfilling prophecies that dominate worldwide markets.

Some of them are rather cute.

The Icarus Factor (the overly ambitious plans of companies - expanding beyond their means - such that they eventually fail in a dramatic manner)

The Santa Claus Rally (general happiness around Wall Street in the period between Christmas and New Year + the pessimists are on holiday)

Of course there're more mundane ones. Sell in May and go away. January Barometer. Triple Witching Day.

Are these prophecies based on empirical observation? Or are they recurrent merely because people believe and hence abide by them? Should one pay no heed at all or accord cautious attention to them?

Really, so many questions and each of them leads on to many more. It is a binary fission of questions with no end in sight.