The relevant definitions within the Employment Rights Act 1996 are:
a week starts on a Sunday and ends on a Saturday[footnote 3]
the holiday pay reference period should start from the last complete working week that was worked ending on or before the first day of leave, starting on a Sunday and ending on a Saturday[footnote 4]
There is an exception for workers whose pay is calculated weekly by a week ending on a day other than Saturday. In these cases, a week is treated as ending with that other day.[footnote 5] For example, if a worker’s pay is calculated by a week ending with a Wednesday, then the employer should treat a week as starting on a Thursday and finishing on a Wednesday.