Format

{
  allMarkdownRemark(filter: { frontmatter: { date: { ne: null } } }) {
    edges {
      node {
        frontmatter {
          title
          date(formatString: "dddd DD MMMM YYYY")
        }
      }
    }
  }
}

Dates can be formatted using the formatString function. Gatsby relies on Moment.js to format the dates. This allows you to use any tokens in your string. See moment.js documentation for more tokens.