Forex trades are done in pairs. You buy one currency while selling another. These currencies have different interest rates, and (if you hold a position into the next day - let it "roll over") the broker should charge or pay you based on this difference. Of course, if you are charged (negative swap), the fee will be larger than if you go the other direction on the same pair and collect money (positive swap). Some brokers charge negative swap no matter if you are long or short on some or all currency pairs. Swap interest is charged or payed at the end of the day, typically around 5 pm New York time. Since there are 7 days in a week, but only 5 trading days, most brokers charge/pay triple swap on Wednesdays. A few charge/pay double swap on Wednesday and Thursday. In some cases, it is possible to get a "swap free" account.