The triple Swap, or 3-day Swap, happens on Wednesdays. On Wednesday, the rollover amount is tripled to compensate for the following weekend (during which time rollover is not charged because trading is stopped at weekends).
When placing a trade in the spot forex market, the actual value date is two days forward, for instance, a deal done on Thursday is for value Monday, a deal done on Friday is for value Tuesday, and so on.