If your payments are due on Monday and are done via the BACS scheme, if the funds appear on a Saturday this is entirely under the banks descretion to credit your account before they recieve the money.
The BACS payments are only processed Monday to Friday. If the second day is a Friday and after 11am the bank will know they will recieve your funds on the Monday and so can choose to credit your account on the Saturday, before 11am your payments may be cancelled by your employer but afterwards you’d have to submit a refund request.
So essentially the bank knows on Friday after 11am they will recieve x funds on Monday so will be nice to you and give you your funds on Saturday instead of Monday.
Just so you know all banks consider Monday as Monday and Saturday as Saturday, they don’t fuzz the lines between days and consider a Saturday as a Friday.