A spot currency contract is the equivelant of a 2 day futures contract. This means that if rollover didn't seamlessly occur you would you have to sell(buy) your existing contract and buy(sell) your new contract every 2 days or face delivery of a big pile of money. Rollovers generally occur around 5 pm EST because that is consdiered the end of the US session and the beginning of the Asian session. Triple interest is paid on wednesday at 5 PM EST because it covers the weekend markets when brokerages and banks are not open. Friday at 5 PM is two days later than Wednesday. This offsets the trades that would have occurred on the weekend.