Crontab uses OR when both day of month and day of week specified

When a crontab line contains both day of week and day of month cron uses OR for the date to run the command.

The POSIX standard dictates at http://pubs.opengroup.org/onlinepubs/9699919799/utilities/crontab.html that:

if either the month or day of month is specified as an element or list, and the day of week is also specified as an element or list, then any day matching either the month and day of month, or the day of week, shall be matched