As an employee, time and again you may have to write leave letters in order to get a sanction of leave. These are formal application letters generally addressed either to your immediate boss or HR personnel for consideration.
Needless to say, the tone and format of leave letters should be formal. The opening paragraph must state clearly the reason for writing, e.g. �I hereby wish to apply for leave�� You must also mention the exact number of days and dates of your leave.
Applying for leave without a valid reason can reflect badly on your professionalism. So, leave letters must include genuine reasons. If your reasons are too personal to be elaborated in an official letter, it is good to mention as much.
If you are seeking leave for a long period, then make sure to include details of the current projects/issues you are working on, their status and also how you would be making up for the lost time. This kind of additional initiative from your end can show you in a good light.
Extend a sincere thank you as you are signing off.