The crontab file is used to schedule commands. To use it you must remember the format it expects. It is not how shall we say mnemonic. In trying to remember the format, I came across a good linuxjournal article that had an easy way to remember it.
Add a header comment that provides the proper sequence:
$ crontab -e |
# ****************Roots crontab*********************** # minute (0-59), # hour (0-23), # day of the month (1-31), # month of the year (1-12), # day of the week (0-6, 0=Sunday), # command |