Trades placed by clients in the spot forex market are settled in two days and open positions held at time of rollover are automatically rolled over by the clearinghouse to the next settlement date. In simplest terms, the open position is exchanged (swapped) for a new position expiring the following settlement date at 5pm EST rollover. This process is also known as "tomorrow, next day" or simply "tom next."

The two positions that are exchanged during rollover are generally not valued at the same price. The difference in value is based on the difference of overnight bank interest rates between the two currencies traded. If the trader is long the currency bearing the higher interest rate then the trader should receive a small credit in his account. Conversely, if the trader is short the currency bearing the higher interest rate then the trader’s account is debited. The nominal debit or credit is reflected in the price of the new position assigned during rollover. This is why you will notice a small difference in price from the original position you had before rollover and the new position assigned during rollover (depending on whether you received a debit or credit).

Wednesday rollover is used to compensate for Saturday and Sunday interest that is unaccounted for while the markets are closed on those two days. Any open spot positions held at rollover on Wednesday will experience three days worth of credits or debits in the account.

The example below is for educational purposes only.

Rollover or Premium example:
If one is long 100,000 EUR/USD at rollover, EUR/USD at rollover is trading at 1.1800, EUR short-term interest rate is 2.25% and the USD short-term interest rate is 4.00%, the theoretical rollover calculation would be as follows:

Formula
Contract notional value x (base currency interest rate - quote currency interest rate) / 365 days per year x current base currency rate = daily rollover interest debit/credit

Therefore: 100,000 x (2.25% - 4.00%) / 365 x 1.1800 = daily rollover interest debit/credit
Further: 100,000 x -1.75% / 365 x 1.1800 = -$5.66 rollover debit to your account

Since one is long a base currency (EUR) bearing a lower interest rate than the quote currency (USD), one pays rollover or premium.