FNB ******* deducting money

I have a personal loan with FNB and have debit order for the 25th of the month. But they deducted the money whenever they want. In the past when a debit order cannot processed on the date, it was deducted the next working day. I have saved some money to buy my children some christmas presents. The next moment on the 20th of December 2019 the debit order was deducted and spoiled my whole christmas. My salary is paid on the 25th of the month and they debit my account on the 24th of Janurary 2020. This had an effect on my credit score and my overdraft facility because i did not have the money. I have put in a query with them, but is not satisfied with their answer. Their greed for money are intolerable!!!