The rule that moves a holiday when it falls on a rest day — where a country has one.
When a holiday lands on a rest day, some countries move it and some do not. The rule that moves it is substitution, and it is the single largest source of wrong dates in calendars that hold dates rather than rules.
The rules differ in more than whether they exist. Most move a holiday forward to the next working day. Some move it backward. Colombia moves a whole class of holidays to the following Monday whatever day they nominally fall on, under the law popularly called Ley Emiliani, which is why Colombian dates drift by up to five days from their nominal ones.
Because substitution is a rule about a rule, it cannot be inferred from a list of dates. A calendar that ships dates has already applied somebody's substitution assumptions and cannot tell you which.
Each of these holidays is not observed on its own date. The nominal date is what the statute names; the observed date is where the substitution rule puts it.
| 🇨🇴 | Monday 11 January 2027Colombia | EpiphanyNominally 6 January, a Wednesday. Colombian law moves this class of holiday to the following Monday — five days later. |
| 🇳🇿 | Monday 4 January 2027New Zealand | Day after New Year's DayNominally 2 January, a Saturday, so it moves forward to the Monday. |
| 🇨🇩 | Saturday 16 January 2027DR Congo | Patrice Lumumba National Hero DayNominally the 17th, and observed the day BEFORE. Not every rule moves forward. |
The nominal date is what the statute names. Substitution is the rule that decides whether that is the day anyone actually takes off.
Substitution moves a holiday. A bridge day is an extra day taken to join one to a weekend, and moves nothing.
See also nominal date · observed date · public holiday · bridge day · rest day
We hold this as substitution_rule on the observance; nominal_date and observed_date on each computed date. See what we hold.