i want to write a letter inviting my friend to birthday party
All you need to do is give him the pertinent information, such as the date, the time, and theme or dress code (if any).
I'd be happy if you could come to my birthday party on January 21. The festivities will start around 6:00 at my house (123 Sesame Street, call me if you need directions) and there will be plenty of food and drink, so bring your appetite! It's going to be very casual, so leave the tuxedo at home. I look forward to seeing you there!