Must I specify my employees' holiday entitlement?

You should include details of your employees' holidays and holiday pay entitlement in:

your employees' written contracts, where they exist
a written statement of employment particulars which you must give to each employee

The written statement is required by law: you must give it to each employee no later than two months after their employment starts. It should contain enough detail to allow each employee's entitlement to be precisely calculated, including any entitlement to accrued holiday on termination of employment.