Answer:
Hey..
Congratulating someone for the success he has achieved makes the person feel happy.
If you are writing a letter to congratulate a colleague,appreciate his hard and dedication.Express your happiness using your words..
When you are writing a Congratulation Letter for Job Promotion,you can use formal or informal style.If your friend has got a promotion,you can write a letter following a demand to throw a party.
Be genuine and appreciative in tone while writing a Congratulations Letter..
Refer the link below to get Sample Congratulation Letter for Job Promotion.