EDIT2: I'm not sure if the effect on weekends is a dip or a pump. the cause is that the traditional banking world does not process FIAT money inflow and outflow to the crypto exchanges and therefore the circuit is broken on weekends.
Definitely a dip. Probably for the reason you mention. Since the peak in price I think every weekend has been a dip, though it does usually bounce back towards the end of the Sunday.
My theory is that during a bull market weekends pump, while during a bear market they dump. I have no data or bothered even checking to see if that is true so take it with a grain of salt.