Can an employee choose to start their maternity leave on any day of the week?

Yes, it is for the employee to choose the day on which their maternity leave will start. It does not have to begin on a particular day of the week, or on a day on which the employee would otherwise have been scheduled to work.

The earliest date that an employee can choose to begin maternity leave is the Sunday at the beginning of the 11th week before the expected week of childbirth.