reopened from #806

Summary:

Celery task does not run when crontab day of week is set to 7.

• Celery Version: 5.4.0
• Celery-Beat Version: 2.6.0

Exact steps to reproduce the issue:

1. Register a task that runs at any desired time on Sunday.
2. Crontab's day of week should be set to 7.
3. Celery task doesn't run.

Detailed information

django_celery_beat/models.py

• The help_text of day_of_week in the CrontabSchedule model says 'Cron Days Of The Week to Run. Use "*" for "all", Sunday '
  'is 0 or 7, Monday is 1. (Example: "0,5")').
  

celery/schedules.py

• However, description presented in Celery module indicates 0 is translated to Sunday.
  
• In the remaining_delta function as well, Sunday is calculated as 0, not 7
  

The comments in django_celery_beat should be modified to reflect this information, or the source code should be modified to ensure the task is sent on Sunday even when the crontab is set to 7.