Cronjob runs before the scheduled time, what could be wrong?

That Cron expression translates to:

At 21:00 on the 19, 20, 21, 22 and 23rd of every month and every Saturday.

So it explicitly told cron to run on Friday the 20th. This is because of:

When the schedule specifies both date and weekday, they're combined with a logical OR,
i.e. the job will run if current_minute == scheduled_minute 
&& current_hour == scheduled_hour && current_month == scheduled_month && 
(current_day == scheduled_date OR current_weekday == scheduled_weekday).

This information is from this handy Cron tool: http://crontab.guru/

To make your job to run on given days when it is Saturday you could use:

00 21 19-23 * * test $(date +%u) -eq 6 && command

This solution is from crontab day of week vs. day of month?