The letter of congratulation for birthday must be written in a linguistic context as close as possible with the addressee.
This example is written in an informal form, thought it to be sent to a friend or a nearby relative. In no case it must be used to be sent to someone with whom let's want to have a more formal relationship.
If we are going to send the letter by post ordinarily, we must calculate the time of delivery in order that it does not come late.
I want to send my more sincere letter of congratulation for this special day for you, and I hope that you spend the best day surrounded with yours.
Many happy returns and happy Birthday!
A big hug,