The information isn't official yet, but if we refer to previous years, we notice that the FIFA demo comes out more or less at the same time.
- FIFA 20 - demo on 10/09 (Tuesday) - release on 27/09
- FIFA 19 - demo 09/13 (Tuesday) - release 09/28
- FIFA 18 - demo on 12/09 (Tuesday) - release on 29/09
This year, we know FIFA 21 will be released a bit later than usual, on October 9, 2020.
If we take the same gap as in previous years between the demo release and the full game release, the FIFA 21 demo should be available between September 20 and 22, 2020.