Reminder of the format... mm hh D M DoW
mm - the minute, a value between 0 and 59
hh - the hour, a value between 0 and 23
D - day of the month, between 1 and 31
M - month, a value between 1 and 12
DoW - day-of-the-week, 0-6 (0 = Sunday)
Each of the five items can be more complex...
A '*' means use all values.
Multiples: e.g. every 20 mins would be '0,20,40'.
Range: e.g. hourly in the evening '19-23'.
Every nth '0-23/n', '*/2' would be every other.
'*/1' is generally acceptable elsewhere, but is flagged here as possibly an unintended entry.
Month can be just the first three letters of the name,
so 'mar,jun,sep,dec' or '3,6,9,12' or '3-12/3'
would all give you quarterly reports.
The DoW can also be e.g. 'mon,wed,fri'.
It is DoW OR DoM (rather than DoW AND DoM), so e.g.
'11th OR a Tuesday' rather than '11th AND a Tuesday'.
Cron Sandbox & Tutorials
There are many crontab tutorials on the web that offer a more detailed explanation.
Come back here to experiment with the various combinations of crontab parameters.