How to setup cron job to run every 5 days?

0 0 */5 * *  midnight every 5 days.

See this, similar question just asked for every 3 days. Cron job every three days

Bear with me, this is my first time posting an answer... let me know if anything I put is unclear.

I don't think you have what you need with:

0 0 */5 * * ## <<< WARNING!!! CAUSES UNEVEN INTERVALS AT END OF MONTH!!

Unfortunately, the */5 is setting the interval based on day of the month. See: explanation here. At the end of the month there is recurring issue guaranteed.

1st  at 2019-01-01 00:00:00
then at 2019-01-06 00:00:00 << 5 days, etc. OK
then at 2019-01-11 00:00:00
...
then at 2019-01-26 00:00:00
then at 2019-01-31 00:00:00
then at 2019-02-01 00:00:00 << 1 day WRONG
then at 2019-02-06 00:00:00
...
then at 2019-02-26 00:00:00
then at 2019-03-01 00:00:00 << 3 days WRONG

According to this article, you need to add some modulo math to the command being executed to get a TRUE "every N days". For example:

0 0 * * *  bash -c '(( $(date +\%s) / 86400 \% 5 == 0 )) && runmyjob.sh

In this example, the job will be checked daily at 12:00 AM, but will only execute when the number of days since 01-01-1970 modulo 5 is 0.

If you want it to be every 5 days from a specific date, use the following format:

0 0 * * *  bash -c '(( $(date +\%s -d "2019-01-01") / 86400 \% 5 == 0 )) && runmyjob.sh