Triple swaps occur because the Spot FX market is a T+2 settlement, which means technically any trades still open past Thursday 00:00 GMT +3 (5pm New York Time) will be held on Saturday and Sunday from a settlement perspective and therefore the cost or benefit of holding those is taken into account on the Wednesday close/Thursday open
In the case of CFDs on Indices, triple swaps occur at Friday 5pm (MT4 server time 00:00) GMT+3and is for Saturday and Sunday's trading.
It is important to also point out that this works both ways. You will receive triple the swap if you were short EURUSD today.
Swap rates can change very quickly depending on market conditions so be sure to check these before you enter your trade (under "Specifications" on the platform) and be mindful of the swap costs on the Wednesday market close (Thursday morning Asia time) and in the case of Indices, Friday close (Saturday morning Asia time).