This is a rejection of a holiday request.
Subject Line: Re: Holiday Request
Dear [Title and Name of Recipient],
Thank you for submitting your holiday request as per company policy.
Unfortunately, due to [time of year / overbooked dates], I am unable to approve these dates for your holiday leave. I apologize for any inconvenience this causes.
Please resubmit your holiday leave with alternative dates. If you would like to know which dates are available, please let me know and I will send you a list.
Best regards,
[Full Name]
"If you would like to know which dates are available, please let me know and I will send you a list." - Instead of saying that at the end, you might attach something like a calendar or the dates that are available, so when you attach it in the email, you can say "I have attached this for your reference".