The date a holiday is nominally on, before any rule moves it.
The nominal date is what the instrument says: Epiphany is 6 January, Christmas is 25 December, the national day is whatever the constitution names. It is the date the rule produces before substitution, in-lieu or bridging rules act on it.
It matters because it is frequently the date people say and rarely the date they take. A document naming 6 January and a payroll system paying on the 11th are both correct about Colombia and are talking about different fields.
The corpus stores both, on every computed date, because collapsing them loses the ability to explain a date — and an unexplainable date is one a customer cannot check.
The nominal date is what the rule names. The observed date is when it is actually taken, after substitution.
Substitution is the rule. The nominal date is its input and the observed date its output.
See also observed date · substitution · public holiday
We hold this as nominal_date on each computed date. See what we hold.