This page has been replaced by the newer Rich Text Page. Convert your existing Invitation Page to the snazzier Rich Text Page with one click by using the button on the right hand side of the editor.
The Invitation page is often used for basic and logistic information -- event name, date, time, location, and directions. You can rename it from Invitation to say whatever you want, in case you're not having an event.
Edit the invitation by clicking the "Edit" button in the top right corner.
Then fill in whatever information you'd like to see.
Don't worry -- if you don't want to include any of the fields, just leave them blank and they will not appear.