If you are planning a birthday party for yourself or one of your friends, you need invite people or more specifically, friends of the birthday person who will celebrate this occasion with him. You can either invite the friends by calling them but when you want to be more formal, you can invitation letters for the birthday party. This is a simple letter in which you mention the person whose birthday you want to celebrate along with the specification of the birthday party i.e. date, time, venue, number of people who will be there, foods and other planning of the party.