Cron on the 31st: 0 0 31 * * only fires seven times a year

Cron matches a date against a pattern, and a date that does not exist can never match. There is no 31 February, so the expression is not an error and the job is not deferred: February simply passes without a run. The same is true of April, June, September and November, which leaves seven months a year.

This is the single most common way a monthly job silently loses runs. The crontab looks right, the expression parses, the job works every time it runs, and five months a year nothing happens. Because there is no error and no log line, the gap is usually discovered by somebody asking why a report is missing rather than by any monitoring.

Day 30 is a milder version of the same problem: it exists in eleven months and never in February, so a monthly job pinned to the 30th misses one run a year. Day 29 exists in eleven months plus February in a leap year, so it misses one run in three years out of four.

If what you wanted was the end of the month, none of these is the right expression, because the end of the month is a different date in different months. The workable approach is to schedule the job across the possible last days and let the script check whether tomorrow is the first, which is covered on its own page.