When you send a payment from your bankaccound to another, the money is immediatly withdrawn, but takes 1 day(same bank) or 2-3 days to shop up on the other accound.
The transaction takes microseconds. The reason it takes two days to shop up, is because thats two days your accound is lower and the other doesnt have the money yet, so a lower figure they pay interest for.
During holydays, banks tend to increase another day. And make exponentially higher profit