How Crontab works

Crontab -e

MIN Minute 0-60
HOUR Hour [24-hour clock] 0-23
MDAY Day of Month 1-31
MON Month 1-12 OR jan,feb,mar,apr …
DOW Day of Week 0-6 OR sun,mon,tue,wed,thu,fri,sat
COMMAND Command to be run Any valid command-line

# MIN   HOUR   MDAY  MON  DOW   COMMAND 
   5         *         *       *      *      echo 'Hello' 
#Run command at 7:00am each weekday [mon-fri]
00 07 * * 1-5 mail_pager.script 'Wake Up'

#Run command on 1st of each month, at 5:30pm
30 17 1 * * pay_rent.script

#Run command at 8:00am,10:00am and 2:00pm every day
00 8,10,14 * * * do_something.script

#Run command every 5 minutes during market hours
*/5 6-13 * * mon-fri get_stock_quote.script

#Run command every 3-hours while awake
0 7-23/3 * * * drink_water.script