Not happy with change on debit order date

Hi, I have a personal loan with FNB and the debit order date is the 25 of every month unless the 25 fall on a weekend..

For the past few months FNB is debiting me account whenever they feel like. The fact that they can see that there is money in my account it doesnt give them a right to debit my account prior to the 25 as per the agreement.

Currently not happy with this and can someone please call me to sort it out .