Why Day-of-Month and Day-of-Week Behave Differently in Cron

The two day fields in a cron expression look independent, but they are not. How they combine is one of the least intuitive parts of cron, and the rule changes from one scheduler to the next. An expression that behaves exactly as you expect in one place can run far more often, or far less often, in another.

Unix cron: either field matches

In Vixie-style Unix cron, if both day-of-month and day-of-week are restricted, a day matches when either of them matches. So this expression is not 'Friday the 13th':

0 0 13 * 5    # midnight on the 13th of every month AND on every Friday

The word 'and' in that comment means 'both kinds of day count', not 'both conditions must hold'. In October 2026 it fires on the 2nd, 9th, 13th, 16th, 23rd and 30th: all five Fridays plus the 13th, which falls on a Tuesday.

There is a subtle exception. If either day field begins with an asterisk, the rule flips to 'both must match', and the restricted field effectively decides. That includes stepped asterisks, so */2 in the day-of-month field counts as 'starting with *'. The expression 0 0 */2 * 1 therefore matches Mondays that fall on odd-numbered days, not every Monday plus every other day.

Spring: both must match

Spring's cron support evaluates every field, so a day matches only when both the day-of-month and the day-of-week match. The same fields written for Spring really do mean Friday the 13th:

0 0 0 13 * 5    # Spring: midnight only when the 13th is a Friday

From October 2026 the next matches are 13 November 2026 and 13 August 2027.

Quartz and EventBridge: one of them must be ?

Quartz and AWS EventBridge avoid the question. Exactly one of the two day fields must be ?, meaning 'no specific value', and the other field alone chooses the days. Setting both is rejected rather than interpreted.

0 0 12 ? * MON-FRI    # Quartz: weekdays
0 0 12 15 * ?          # Quartz: the 15th of each month
0 0 12 * * MON-FRI     # rejected by Quartz: neither field is ?

Summary

Working around it

• To get 'AND' in Unix cron, schedule on one field and test the other in the job, for example run on the 13th and exit unless the weekday is Friday.
• To get 'OR' in Spring or Quartz, define two schedules.
• Prefer weekday names so the weekday part is never misread through numbering differences.

The Cron Builder on DevCraftKit applies the rule of the dialect you select, explains in plain English how the two day fields interact for your expression, lists the matching dates, and warns when switching dialect changes the upcoming run times.

Conclusion

Whenever an expression restricts both day fields (see also five-field vs. six-field cron), stop and look up your scheduler's rule. The same text can mean a union in one place, an intersection in another, and an error in a third.