Remember how you learned that 一周 means "one time around?" Well if you're mourning one time like this a year, then it's because it's the first anniversary of death for a family member. Maybe it was the death of your mom, your grandad or your goldfish, this is the first year since they died.

This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.