A cron is a service on Unix servers that allows tasks to be scheduled and executed automatically. The cron application runs constantly and will, according to instructions, carry out its orders. These orders are stored in a file called crontab. This file is a to-do list that contains lines that might look like this:

30 22 * * * lynx --dump http://www.kiran.org.in > /dev/null

The crontab format dictates that each line contain six fields separated by spaces or tabs. The first five fields represent, in order, minutes, hours, days, months, and day of the week (from 0 to 6, with 0 being Sunday).
Notice that you can specify the day of operation as either a day of the month (1–31) or a day of the week (Sunday through Saturday), the latter being date-indifferent.