A bill becomes due on the due date.
However some bills may allow and offer a grace period of between 2 days to 7 days after the due date on the bill to allow you time to pay the bill.
If you fail to pay the bill after that grace period then a late fee will usually apply or your services may be terminated.
For example if you have an internet bill and the bill is due on the 25th of the month then it needs to be paid by then and if not a late fee can apply.
That is unless there is a grace period.
Not all companies offer a grace period.
My internet provider does not offer a grace period and my internet bill is always due by the 25th of the month.
If I don't pay the bill by the 25th of the month then a late fee of $16.00 is applied to the bill.
Then if I don't pay after a few months my internet gets shut off.
With an electric bill you sometimes have a few weeks past the due date to pay the bill our the electric will be shut off.
It all depends on the company but the bill is always due by the due date on the bill.