If the NBA decided to begin the 20-21 season on Christmas Day, sources tell Vincent Goodwill of Yahoo! Sports that training camp would begin during the first week of December.
The NBA could have an accelerated free agency immediately after the draft on November 18th.
The league had a similarly short free agency period coming out of the lockout in 2011 with the season beginning on December 25th. Free agency officially began that season on December 9th.