Sellers have broken down $18/BTC on MtGox... right on schedule according to weekend drops.
There had been a pattern that was pretty reliable:
If on Wednesday night / Thursday morning the high water mark for the prior 7 days was below the high for the week before, then a weekend dip could be counted on where selling and then buying those coins back on Sunday afternoon would yield 10 to 15% gain.
The reason that was happening was likely due to the outflow spigot being wide open (bitcoins can be sold and funds withdrawn 24x7) yet at the same time there is no corresponding inflow in which funds can be added to the exchange.
Funds added to Dwolla and by bank wires arrive only on business days, so there are no new funds arriving from those sources on the weekend.
With a lower price come Sunday, those in Asia and Europe are the first who are able to wire funds or make SEPA transfers, so once that happens on the following week, the market price would once again rise. So to reaquire those bitcoins, the repurchase would need to occur by late Sunday.
Whether or not this pattern recurs is unknowable, but with a higher difficulty, miners trying to pay off mining equipment just had their "income" cut by a third from this latest adjustment. As a result many will likely be hoarding less (if at all) and selling more of what their mining activities bring in.
At some point there will be a futures market with options where these ultra short-term (weekly) cycles will be leveled, but for now the weekend dip appears to make the "sell Wednesday night/Thursday morning and buy back on Sunday" trade continue to be profitable.
Though don't forget -- if there is a big buyer, that amounts sold by miners affects the market price very little. See the chart from Easter weekend to help understand that situation more clearly.
