Workers are entitled to 5.6 weeks paid holiday per year (capped at 28 days, so people who work 6 days per week don't get any more holifday than those who only work for 5 days).
The rate of pay to be paid during those holiday periods must be the same as the employee's usual rate of pay. Where an employee works irregular hours (or otherwise gets a varying rate of pay, such as through the inclusion of bonuses), the rate of pay is calculated by totalling the pay from the past 12 paid weeks (which might not necessarily be consecutive) and dividing by 12 - thus giving 1/12 (one twelth) of the total.