A day nearly everybody takes off that no instrument names.
A de facto holiday is one that exists in practice and in no instrument. Nothing declares it, no gazette publishes it, and most people are absent: the days between Christmas and New Year in much of Europe, the Friday after Thanksgiving in the United States, the working days inside a national festival week.
They are invisible to any calendar built from published sources, which is most calendars, and they are frequently more disruptive than the declared holidays around them — because a plan accounts for the declared ones.
This corpus does not hold them, and the reason is the rule the whole estate runs on: we record what an authority published. A de facto holiday has no authority to cite, and recording it would mean asserting a claim about attendance we cannot evidence.
An observed-but-open day is recorded because an authority names it and it closes nothing. A de facto holiday is named by nobody.
A bridge day is a specific kind of de facto holiday — the working day between a holiday and a weekend.
See also bridge day · observed but open · public holiday · privilege holiday
We do not hold de facto holidays. Recording them would mean asserting an attendance claim with no source, which is the one thing this corpus refuses. See what we hold.