Here is an example of invitation letter:
Brgy. San Lorenzo
Valenzuela City 1143
November 8, 2014
Do you know that in Saturday I will be celebrating my birthday?
My eldest sister is giving an informal party to celebrate it. She ask
me to invite my friends or classmates.
The party will be held at our home starting at three o'clock in the afternoon. I hope you will come. I'll be expecting you.
--Mizu <--(Don't copy this)