Date: __/__/_____ (date)
My dear brother/sister,
I hope you are doing well and I am writing this letter to inform you that I am turning _______ (age) on the coming __/__/____ (date) and I would like to invite you to the birthday celebration which is to be celebrated at ____________ (mention venue).
You are hereby invited and requested to be present at the said location at __:__ (time). I would enjoy your company and also it has been a long time since we have met. I await your kind response.
_________ (Mention name)