No official word has been revealed about the actual FIFA 19 demo release date, but according to the site TrueAcheivements, the demo will be released on September 13. The site doesn't actually reveal how they got that info, but they seem pretty sure about it.
No further details were revealed either, but most assume the FIFA 19 demo will include Kick-Off matches (and the new corresponding modes), a sample for Alex Hunter's Journey, and the new Champions League licence as the big highlight.
EA Sports should release official details on the demo soon.