Are stock prices likely drop off a little bit on a given friday afternoon?
There are classes of 'traders' who close their positions out every evening, not just on fridays.  But their are other types of businesses who trade shortly before or nearly right at market close with both buys and sells  There are lots of theories as to how the market behaves at various times of day, days of the week, months of the year.  There are some few patterns that can emerge but in general they don't provide a lot of 'lift' above pure random chance, enough so that if you 'bet' on one of these your chances of being wrong are only very slightly different from being right, enough so that it's not really fair to call any of them a 'sure thing'.  And since these events are often fairly widely spaced, it's difficult to play them often enough to get the 'law of large numbers' on your side (as opposed to say card-counting at a blackjack table) which basically makes betting on them not much different from gambling