Cron every 3 days: 0 0 */3 * * and what happens monthly

The walk starts at day 1 and adds three, giving ten days in every month and an eleventh, day 31, in the seven months that have one. Because the day of month field never counts past the end of a month, the pattern restarts on the 1st regardless of where the previous month stopped.

That restart produces three different boundary gaps depending on the month. After a 31 day month the last run is the 31st and the next is the 1st, one day later. After a 30 day month the last run is the 28th and the next is the 1st, three days later, which happens to be correct. After a 28 day February the last run is the 28th and the next is 1 March, one day later.

So the schedule is every three days for most of the month and something else at the seams, seven or eight times a year. For a cleanup job or a report that just needs doing regularly that is entirely fine. For anything that computes a window of the last three days it means overlapping or duplicated coverage at every boundary.

The alternative is the same one that fixes every other day: run daily and guard on the epoch day number modulo three. That gives an exact 72 hour cycle that ignores month lengths, at the cost of two thirds of the scheduled runs doing nothing except exiting.