Why a cron job runs on the wrong day
When both day fields are set, standard cron runs the job if either one matches. See which systems do that, how to test the rule, and how to require both.
Guides
One question at a time. Each guide explains the answer, shows an example, and opens the browser tool that does the work.
What cron expression runs every 5 minutes?
*/5 * * * * runs at minutes 0, 5, 10, and so on. See every 10, 15, and 30 minutes, plus hourly and daily cron, and the mistakes that move the run.