In Pages, it's called a *Placeholder Text*, and it's easy to create. Highlight "Month Day, Year" in your document and go to *Format > Advanced > Define as Placeholder Text* or press
Ctrl–Option–Command–T if you are a shortcut person.
You won't notice any difference to the text, but if you single-click on it, it'll be highlighted and enable to type in the date that you like.
If you want to change the text in the placeholder (say, if you want it to display "Enter the date here"), simply go to *Format > Advanced > Enable Placeholder Text Authoring* and the placeholder will have a little blue border around it when you click it for editing. To "lock it back", simply *Disable Placholder Text Authoring*.