WhenMathBlackout Dates Are Not the Same List as Public Holidays
A project blackout date is a separate, deliberately chosen exclusion list, not a subset or superset of public holidays.
Public holidays (covered in their own resource above) are a jurisdiction's standard non-working dates. A blackout date is a different, purpose-specific kind of exclusion: a deliberately chosen date or range where a specific activity should not be scheduled, such as a change freeze during a peak business period, a facility closure for maintenance, or an individual's known unavailability. Blackout dates frequently include dates that are not public holidays at all, and can just as easily exclude a date that would otherwise be a normal working day for an entirely different reason.
Why the two lists serve different purposes
A public holiday list answers "which dates does this jurisdiction treat as a standard non-working day," and is maintained externally (a government calendar), applying broadly to anyone operating in that jurisdiction. A blackout list answers a narrower, purpose-specific question, such as "which dates should this particular project, system, or person avoid," and is maintained internally by whoever owns that specific constraint, independent of any jurisdiction's holiday calendar.
Why blackout dates often do not align with holidays at all
A retail company's change-freeze blackout typically covers its own peak sales period, which may have no relationship to any public holiday. An individual's personal blackout dates (a planned vacation, a known unavailability) are inherently unrelated to any jurisdiction's holiday calendar. Treating a blackout list as if it were derived from, or a subset of, public holidays will miss exactly the dates a blackout list exists to capture in the first place.
How WhenMath keeps the two settings separate
The Blackout-Date Deadline Planner accepts an explicit, independently entered blackout date list layered on top of (not derived from) any selected public holiday calendar, and the Custom Excluded-Date Calculator provides the same free-form exclusion-list input for business-day counting more generally, so a deliberately chosen blackout window is never confused with, or assumed to already be covered by, a jurisdiction's holiday exclusions.
Key takeaway. A blackout date list is a separate, deliberately maintained exclusion list for a specific purpose, not a variant of a public holiday calendar. Enter it explicitly rather than assuming holiday exclusions already cover it.
Frequently asked
- Can a blackout date and a public holiday be the same date?
- Yes, coincidentally, but that overlap is not required or assumed. A blackout list stands on its own regardless of whether any of its dates also happen to be public holidays.
- Should a blackout list include weekends explicitly?
- No, weekends (or a custom workweek's non-working days) are already handled by the separate workweek setting covered in its own resource above, so a blackout list only needs to contain the additional, purpose-specific dates beyond the standard non-working pattern.
- Does a blackout date silently push a deadline, or does it require confirmation?
- The Blackout-Date Deadline Planner shows exactly which blackout dates were skipped and how they shifted the computed deadline, so the effect of the blackout list is visible in the result rather than applied silently.
Related engines
- Blackout-Date Deadline Planner -- Plan a business-day deadline around your own unavailable date blocks.
- Holiday Impact Calculator -- See how many working days a region's public holidays remove.
- Custom Excluded-Date Calculator -- Count business days after applying your own excluded dates.
Last updated 2026-09-04