The markdown header is the metadata for your post or page.
Let's check all available options harmonic have:
| layout | The layout of the file (post|page) - NOT IMPLEMENTED YET* |
| title | The title of your post or page |
| date | The date in JSON format (Ex: new Date().toJSON()) |
| comments | If the page or post have comments enabled - NOT IMPLEMENTED YET* |
| published | If the page or post is available online - NOT IMPLEMENTED YET* |
| keywords | The keywords of the page or post - NOT IMPLEMENTED YET* |
| description | The post/page description |
| categories | The categories |
| authorName | The authorName |
NOT IMPLEMENTED YET means the default theme doesn't support those features.