skeleton for “crontab -e”:

SHELL=/bin/sh
PATH=$HOME/bin:/bin:/usr/bin:/usr/local/bin:/usr/sbin/

#+------------------- Minute:       0 - 59
#|  +---------------- Hour:         0 - 23
#|  |  +------------- Day of Month: 0 - 31
#|  |  |  +---------- Month:        0 - 12 (or names:  3-letter abbrev.)
#|  |  |  |  +------- Day of Week:  0 -  7 (0 or 7 is Sun, or use names)
#|  |  |  |  |
#V  V  V  V  V

Older unix systems tended to employ individual crontab files.
More often now, Linux systems employ /etc/crontab which invokes anacron to run scripts in the following directories:

/etc/cron.hourly
/etc/cron.daily
/etc/cron.weekly
/etc/cron.monthly

Good packages usually have cron scripts for one of these.