anniversary of a person's death (on which Buddhist commemorative rites, etc. are performed)
49th day after a person's death, on which Buddhist rituals are performed
「忌日」 has another reading with different meanings.
祖母の忌日に家族で墓参りをした。
We visited the grave as a family on my grandmother's death anniversary.
忌日には毎年法要を営んでいる。
A memorial service is held every year on the anniversary.
作家の忌日に代表作を読み返した。
On the author's death anniversary, I reread their masterpiece.