Themas is perfectly correct but only as long as you work 5 days per week. The current rule is that the minimum holiday entitlement is 4.8 times the number of days you work each week (subject to a maximum of 28 days per year)
As Themas indicates, your holiday entitlement includes any days when the employer closes the business (e.g. due to public holidays).
As from 1 April next year, the calculation will be 5.6 times the number of days you work each week (subject, once again, to a maximum of 28 days).
So anyone who works 5 days per week is entitled to a minimum of 24 days paid holiday this year. In 2009, they're entitled to a minimum of 27 days paid holiday. (That's because one quarter of the year is under the current rules and three quarters of the year will be under the new rules). From 2010, their entitlement will be 28 days per year.
http://www.direct.gov.uk/en/Employment/Employe es/WorkingHoursAndTimeOff/DG_10034642
Chris