When creating a calendar, your photo options aren’t limited to just the page above each month. You can also add photos to individual dates. To commemorate a birthday, add a photo to the birthday girl’s date. To never forget your anniversary, put a wedding photo on the date. (Another good way to never forget an anniversary is to forget it just once, but this method is not recommended.)
To add a photo to a date, drag it from the Photos pane to the calendar date.
To remove a photo from a date, select the date and press the Delete key.