How can I explain the usage of the indefinite article in the following sentences?
- Thanksgiving is a day when we celebrate the harvest.
- New Year' Eve is a night when I have fun with my friends.
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When you want to talk about the characteristics of Thanksgiving (e.g., its role in celebrating the harvest), you use the indefinite article "a." Thanksgiving is a particular day, perhaps among several, for harvest celebrations.
When you want to talk about the definition of Thanksgiving, use the definite article "the." Thanksgiving is the day to celebrate the harvest, that's what it's for, and it's the only one.
The question is whether the object is unique or not.
Thanksgiving is a day when we celebrate the harvest.
There may be several days when we celebrate the harvest; Thanksgiving is one of them.
New Year' Eve is a night when I have fun with my friends.
There are several nights when you have fun with your friends (if not, you should get out more). New Years' Eve is one of them.
Christmas is the day when Santa Claus brings presents for the children.
You wouldn't use a here, because there's only one day when he brings presents.