This question is arguing about "the anniversary of the massacre at Tiananmen Square".
In my native language there very similar words for both concepts (anniversary and commemoration), but anniversary is always for a positive event and commemoration is for negative only, so above sentence sounds strange.
I have checked Wikidiff:
Of a significant event, a day that is an exact number of years (to the day) since the event occurred.
The act of commemorating; an observance or celebration designed to honor the memory of some person or event.
So, technically something bad (even involving someone's death) might be the subject of an anniversary, but commemoration typically deals with someone's death (memory).
I am wondering how native English speakers use these concepts.
Question: What exactly is the difference between anniversary and commemoration?