https://help.ubuntu.com/community/CronHowto
The quick and simple editor for cron schedule expressions:
https://crontab.guru/
Edit crontab file:
crontab -e
Crontab lines
Each line has five time-and-date fields, followed by a command, followed by a newline character ('\n'). The fields are separated by spaces. The five time-and-date fields cannot contain spaces. The five time-and-date fields are as follows:
minute (0-59), hour (0-23, 0 = midnight), day (1-31), month (1-12), weekday (0-6, 0 = Sunday).
01 04 1 1 1 /usr/bin/somedirectory/somecommand
The above example will run /usr/bin/somedirectory/somecommand at 4:01am on January 1st plus every Monday in January.
An asterisk (*) can be used so that every instance (every hour, every weekday, every month, etc.) of a time period is used.