The following are defaults in Ubuntu 20.04 LTS.
$ ll cron.* cron.d: total 40 drwxr-xr-x 2 root root 4096 Jun 27 11:14 ./ drwxr-xr-x 148 root root 12288 Aug 9 22:35 ../ -rw-r--r-- 1 root root 285 Jul 17 2019 anacron -rw-r--r-- 1 root root 775 Oct 26 2020 certbot -rw-r--r-- 1 root root 201 Feb 14 2020 e2scrub_all -rw-r--r-- 1 root root 712 Mar 27 2020 php -rw-r--r-- 1 root root 102 Feb 14 2020 .placeholder -rw-r--r-- 1 root root 191 Feb 2 2021 popularity-contest cron.daily: total 68 drwxr-xr-x 2 root root 4096 Aug 10 02:47 ./ drwxr-xr-x 148 root root 12288 Aug 9 22:35 ../ -rwxr-xr-x 1 root root 311 Jul 17 2019 0anacron* -rwxr-xr-x 1 root root 376 Dec 5 2019 apport* -rwxr-xr-x 1 root root 1478 Apr 9 2020 apt-compat* -rwxr-xr-x 1 root root 314 Feb 19 2020 aptitude* -rwxr-xr-x 1 root root 355 Dec 29 2017 bsdmainutils* -rwxr-xr-x 1 root root 384 Nov 19 2019 cracklib-runtime* -rwxr-xr-x 1 root root 1187 Sep 6 2019 dpkg* -rwxr-xr-x 1 root root 377 Jan 21 2019 logrotate* -rwxr-xr-x 1 root root 1123 Feb 26 2020 man-db* -rw-r--r-- 1 root root 102 Feb 14 2020 .placeholder -rwxr-xr-x 1 root root 4574 Jul 18 2019 popularity-contest* -rwxr-xr-x 1 root root 214 Dec 7 2020 update-notifier-common* cron.hourly: total 20 drwxr-xr-x 2 root root 4096 Feb 2 2021 ./ drwxr-xr-x 148 root root 12288 Aug 9 22:35 ../ -rw-r--r-- 1 root root 102 Feb 14 2020 .placeholder cron.monthly: total 24 drwxr-xr-x 2 root root 4096 Jun 27 11:14 ./ drwxr-xr-x 148 root root 12288 Aug 9 22:35 ../ -rwxr-xr-x 1 root root 313 Jul 17 2019 0anacron* -rw-r--r-- 1 root root 102 Feb 14 2020 .placeholder cron.weekly: total 32 drwxr-xr-x 2 root root 4096 Jun 27 11:14 ./ drwxr-xr-x 148 root root 12288 Aug 9 22:35 ../ -rwxr-xr-x 1 root root 312 Jul 17 2019 0anacron* -rwxr-xr-x 1 root root 813 Feb 26 2020 man-db* -rw-r--r-- 1 root root 102 Feb 14 2020 .placeholder -rwxr-xr-x 1 root root 211 Dec 7 2020 update-notifier-common*
See also cron rules with respect to changing system time, explained in this SO answer, and commented in the cron source here.
Some backup scripts, run by administrator:
# m h dom mon dow command 23 4 * * * /usr/backup/backup.sh >/usr/backup/backup.log 23 5 * * 6 /usr/backup/backup-seafile.sh >/usr/backup/backup.log