Special idiomatic expression: If it's one of a kind, there is no other like it.
"I can't remember which one of these expressions to use," the student complained.
"As usual," I replied, "it's all about context."
Here's how it works:
If you say one day, you are not talking about a day among a group of days, but simply a day, any day.
On the other hand, if you use one of, you are speaking of the noun that follows in connection to a group of days; thus, you speak about one of the days you worked last month.
If you eat one chocolate, you eat a single chocolate, not one from a specific group of chocolates.
If you decide to allow yourself only one drink, because you are planning on driving, the emphasis is on the number, one drink only, and not on the group of which that drink is a member.
It's also about grammar. One is followed by a singular noun. One day, one chocolate, one drink.
In contrast, one of is followed by the and then a plural noun. One of the days I worked last month, one of the chocolates I bought at Purdys, one of the drinks on the table.
As you see, the plural nouns above are followed by specifying modifiers that define the group and answer the question "which?" Answer: the one I worked last month, the one I bought at Purdy's, the one on the table.
Beware: the word one can sometimes be plural. In this case, one is preceded by the, and followed by specifying phrases or clauses, like this:
Which days? The ones I worked night shift.
Which drinks? The ones on the table.
Which chocolates? The ones from Purdy's.
Whether you use one or one of the, an adjective can precede the noun. One dark chocolate, one alcoholic drink, one sunny day. Or one of the cream-filled chocolates (if there are any left); one of the alcoholic drinks (as long as it isn't too strong); one of the busiest days we had at work last month.
Remember: one of is ALWAYS followed by the and a specifying modifier.
