Article 'a' with 'night' and 'day': It was opening night

[Tom]

Hi

The following sentences have been taken from a novel. Could yo please tell me where the article is before night and day?

1- It was opening night. (why not an opening night?)
2- It's market day in Cortona. (why not a market day?)

Tom

[Mister Micawber]

.
They are the names of particular nights and days: Christmas Day, parents' day (at school), etc. (Also: tomorrow night, yesterday).

However, they can also appear as the opening night, the market day-- here they are not named days, but descriptive adjectives for the days.
.
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Native English teacher at Mister Micawber's