A rollover is the simultaneous closing of an open position for today's value date and the opening of the same position for the next day's value date at a price reflecting the interest rate differential between the two currencies.
In the spot forex market, trades must be settled in two business days. For example, if a trader sells 100,000 Euros on Tuesday, then the trader must deliver 100,000 Euros on Thursday, unless the position is rolled over. As a service to customers, all open forex positions at the end of the day (5:00 PM New York time) are automatically rolled over to the next settlement date. The rollover adjustment is simply the accounting of the cost-of-carry on a day-to-day basis. .