Dear Sir,
My sister's marriage will take place on 20th of this month. I am requesting leave for 19/03/2009 to 21/03/2009. I would greatly appreciate it if you would approve my request.
Thank you,
Raghu Chowdary
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Where I live, we don't write "Dear Sir" to our boss, since we know his name. If "Dear Sir" is what you write, then you should leave it alone. I would write his name.