Cron

# * * * * *  command to execute
# ┬ ┬ ┬ ┬ ┬
# │ │ │ │ │
# │ │ │ │ │
# │ │ │ │ └───── day of week (0 - 7) 
# │ │ │ └────────── month (1 - 12)
# │ │ └─────────────── day of month (1 - 31)
# │ └──────────────────── hour (0 - 23)
# └───────────────────────── min (0 - 59)

Note:- for days 0 to 6 are Sunday to Saturday, or use names; 7 is Sunday,

To Turn logging on

By default, the logging for the cron demon is not enabled in Raspbian . To enable it, open the file /etc/rsyslog.conf via

$ vi /etc/rsyslog.conf

and uncomment the line

# cron.*      /var/log/cron.log

After that, you need to restart rsyslog via

$ /etc/init.d/rsyslog restart

You can find the cron logs in /var/log/cron.log

Source: Enable crontab logging in Debian Linux

GUI

A GUI  for Cron is available by installing the gnome-schedulepackage:

sudo apt-get install gnome-schedule

You can then launch the program Scheduled Tasks from the main menu.

Good example / info sites for more Cron

http://www.computerhope.com/unix/ucrontab.htm

https://www.raspberrypi.org/documentation/linux/usage/cron.md

https://en.wikipedia.org/wiki/Cron