Why is it a little odd to say "I work at nights"?

It's idiomatically correct to say either of the following, but not both:

I work at night.

I work nights.

Idioms and idiomatic expressions can't be explained very well.

http://www.answers.com/idiomatic

Here are a couple of websites with lists of English idioms. Remember ... these are correct expressions, but usually cannot be explained.

http://en.wikipedia.org/wiki/List_of_English-language_idioms

http://eflnet.com/idioms/idiomlist/all

Saying "I work at nights" might sound a little odd because it is not the traditional way to describe your work schedule. Typically, people would say "I work at night" or "I work nights" to indicate that their job mostly takes place during the nighttime. Here's why:

1. Plural versus singular: The word "nights" refers to multiple nights, implying that you work every night. On the other hand, "night" can be considered more of a general term, expressing the idea that your job primarily occurs during the nighttime hours without specifying the exact number of nights worked.

2. Specific versus general: Using "nights" implies a specific schedule, suggesting that you work all or most nights. This can sound a bit unusual because it is less common for someone to work exclusively during the nighttime.

So, while saying "I work at nights" might still be understood, it deviates from the more commonly used expressions.