The Cambridge Dictionary and the Free Dictionary mention that "par for the course" is to be used with negative events. Is it only used for negative events? If I did something good, for example, could I say it is "par for the course"?
The expression "par for the course" roughly means, "this is normal, so don't be impressed or surprised". It downplays the importance of one particular incident given the fuller context. This means it can be used with negative or positive expressions. It's important to note that it doesn't make sense with neutral expressions. In this sentence, for example, the expression is meaningless:
I just finished my morning tea, but that's par for the course.
Nobody is impressed or surprised that I drank tea this morning because I drink tea every morning, but downplaying it suggests that you should be initially impressed or surprised.
From one of the other answers:
I aced the test, but for me that's par for the course.
Here, the expression is of something good, my success on the test, but the connotation of "par for the course" is that this success is not significant, downplaying it.
It means the same thing when used with a negative expression:
My boss took credit for the work I did on that project, but that's par for the course with her.
Here, the direct meaning is that my boss not acknowledging my work is normal, nothing to be surprised about, downplayed.
Now, the negative aspect that those dictionaries refer to is the implied meaning when "par for the course" is used with negative expressions. In the example about my boss, the subtext is, "I'm complaining about what a terrible boss I have. Not only did she take credit for my work on this project, but this is normal", which of course, is much worse. When used this way, it's very negative.
Even when used with positive-looking expressions, it can have a negative connotation. Another example from the other answers:
Joe won all his tennis matches on Saturday, but that's par for the course for him.
The direct meaning is, "It's not surprising that Joe won all his matches because he always does." Depending on the context and the speaker's tone of voice, there might also be an inferred subtext of, "I'm complaining that he always wins."
Although 'par for the course' is often used with a negative implication (e.g. 'we can't expect anything better'), it is not exclusively negative. It can simply mean 'normal or usual', e.g. 'Joe won all his tennis matches on Saturday, but that's par for the course for him', 'Jimmy cooked a gourmet meal for his friends that rivalled anything that an expensive restaurant could provide, but that's par for the course with him'
Lexico (Oxford Dictionaries) gives a positive example in its definition:
par for the course
What is normal or expected in any given circumstances.
‘looking gorgeous is par for the course with her’
It's much more commonly used with negative occurrences. However, a few dictionaries support the use of par for the course with more positive things to express being unimpressed, for example Longman:
Such service companies want your agency's business and lavish lunches and gifts are par for the course.
That said, when applied to something good you did you should use it cautiously - it can sound rather boastful:
I aced the test, but for me that's par for the course. (I was very sure I'll ace the test and I'm bragging that it's nothing special for me)
No, of course not.
What makes you think "par for the course" has mainly… even, any… negative meaning?
I happen to have heard that expression many hundreds of times, and never once noticed any suggestion of negativity…
"Average or (a bit) above" perhaps.
"Below average" pretty-much never.
Specifically, "par for the course" clearly does mean "on average."