Re: Need Letter to Employee for Working in Off days
With ems's corrections, it appears you want to write to say "thank you" to the person who came in to work extra hours to get a project done. At first I thought you wanted to ask an employee to do this in the future.
A good way to start is to pretend you are talking to the person and then just write down what you would say.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.