Cron log on debian systems

Under Ubuntu, cron writes logs via rsyslogd to /var/log/syslog. You can redirect messages from cron to another file by uncommenting one line in /etc/rsyslog.d/50-default.conf. I believe, the same applies to Debian.

Debian has the default rsyslog.conf under /etc/rsyslog.conf and the cron line is in default file on line 63

Edit:

Good point by Anthony Geoghegan.

Version of my debian is 9 I'm not sure how it is in previous versions

 54 ###############
 55 #### RULES ####
 56 ###############
 57 
 58 #
 59 # First some standard log files.  Log by facility.
 60 #
 61 auth,authpriv.*                 /var/log/auth.log
 62 *.*;auth,authpriv.none          -/var/log/syslog
 63 cron.*                          /var/log/cron.log
 64 daemon.*                        -/var/log/daemon.log
 65 kern.*                          -/var/log/kern.log
 66 lpr.*                           -/var/log/lpr.log
 67 mail.*                          -/var/log/mail.log
 68 user.*                          -/var/log/user.log