The date a holiday is actually taken, after any rule has moved it.
The observed date is when the day off actually falls. It is the nominal date with every applicable rule applied — substitution, in-lieu, bridging — and it is the date that matters operationally, because it is the day nobody is there.
Almost every calendar product publishes only this field, which is reasonable and lossy. It means the calendar can tell you a date and cannot tell you why, and cannot be checked against the instrument that produced it.
Holding both is what makes a date auditable. If our observed date is wrong you can see whether we read the statute wrong or applied the rule wrong, which are different errors with different fixes.
The observed date is where the day lands. The nominal date is where the statute puts it before any rule moves it.
See also nominal date · substitution · public holiday
We hold this as observed_date on each computed date. See what we hold.