From the Commission On Presidential Debates:
Dear Mayor Giuliani: Thank you for your letter dated August 5. We are pleased that President Trump and Vice President Biden both have agreed to participate in the debates the Commission on Presidential Debates will sponsor on September 29 (Cleveland), October 15 (Miami-Town Hall), and October 22 (Nashville).
We also look forward to the vice presidential debate scheduled for October 7 in Salt Lake City. Debate invitations will be extended after the major parties have formally nominated their respective candidates and after application of the Commission’s Non-Partisan Candidate Selection Criteria, as required by applicable law.
In your letter, you express the Trump campaign’s interest in a presidential debate in early September. You state that such a debate is necessary because some states begin sending out mail-in ballots before the first scheduled debate.
There is a difference between ballots having been issued by a state and those ballots having been cast by voters, who are under no compulsion to return their ballots before the debates. In 2016, when the debate schedule was similar, only .0069% of the electorate had voted at the time of the first debate.
While more people will likely vote by mail in 2020, the debate schedule has been and will be highly publicized. Any voter who wishes to watch one or more debates before voting will be well aware of that opportunity.