Hide post title and meta on single posts

Use this to hide the whole post header (title + date, author, categories) on all single posts:

.single-post .hentry .entry-header {
    display: none;
}

To hide it just on one specific post, use the post ID like this:

.postid-144 .hentry .entry-header {
    display: none;
}

Tip: replace 144 with the actual ID of your post. You can find the ID in the URL bar when you edit your post.