Your manager's details
Always address the letter to your manager or boss. They should be someone that can approve the leaves. Start with their full name, position, company name and then company address
The next thing to add are your own details. Make sure to give you complete details including full name, position and contact information.
Any communication in the office should have the date on it and so should your holiday leave application.
It is a good idea to add a subject line so that the reader knows that the rest of the letter will be about.
Address your boss formally using the correct honorary and their last name.
Get down to business right away and mention that you would like to take a holiday leave as part of your annual leave. Also mention the dates that you will leave and return.
You should throw in some details about your vacation, whether you will be out of the state or country.
Obviously when you are on vacation the work will not stop, so yo need to either recommend a person to take over your duties or give another plan to cover for your projects.
Always sign off by thanking your superior and then adding your signature and complete name in print.