1. Make sure you treat the request for holiday fairly and sympathetically using a consistent approach.
2. Is it made clear, either in the employment contract or a staff handbook, that taking holiday is subject to approval and that you reserve the right to refuse holiday at times inconvenient to the company? If not, any decision to refuse a request for holiday will be harder to justify and may amount to a breach of contract.
3. If you feel unable to grant the request, consider the reasons for this before refusing the request to make sure you can justify the decision on objective grounds.