Calculating holiday pay can be a minefield especially following the European cases which mean you have to include commission and overtime (even if it is voluntary but done on a regular basis) in the calculations. At the moment you have to take the average pay (including commission and overtime) over the period of 12 weeks prior to the holiday to work out the pay. From 6th April 2020 the holiday pay reference period will increase from 12 weeks to 52 weeks excluding any weeks where the employee did not receive any pay.
