Are trades held over the weekend affected by trade rollover?

Yes, rollover applies on weekends.

However, spot Forex transactions technically have a 2-day settlement period. Therefore, the weekend rollover is accounted for 2 days later, on Wednesday.

Also, if there is ever a national holiday, where banks in that country are closed, there is generally 0 rollover applied to that countries currency pairs. For example, if there were a US holiday on a Monday, there would likely be no rollover on that Monday, and twice normal rollover on Tuesday.

Please note that the rollover available on the Trading Station II is for the upcoming 5pm ET (New York Time) debit/credit. This means that from 5:01pm ET on Tuesday through 5pm ET on Wednesday, the rollover amount you are going to see is Wednesday's (triple) rollover.