Note: The day of a command’s execution can be specified by two fields — day of month, and day of week. If both fields are restricted (i.e., don’t start with *), the command will be run when either field matches the current time. For example, 30 4 1,15 * 5 would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.
What does it mean precisely? If you specify both the day of month and the day of week field, then cron will run the command when either of the fields match. In other words, there’s a logical OR relationship between the two fields. Let’s look at an example:
This expression translates to “Run at the midnight if it’s the first day of the month OR Monday”. An expression like this could be handy for a job that sends weekly and monthly reports as it probably needs to run at the start of every week, and the start of every month. However, if either field is unrestricted, the logical relationship between the fields changes to “AND”. For example:
Here, the day of month field is unrestricted. Cron will run this command when both the day of month field AND the day of week fields match. Since * matches any day of month, this expression effectively translates to “Run at midnight every Monday”.