der Todestag
death anniversary
The anniversary of the day on which a person died.
An seinem Todestag besuchen wir jedes Jahr sein Grab.
On his death anniversary, we visit his grave every year.

death anniversary / date of death
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death anniversary
The anniversary of the day on which a person died.
An seinem Todestag besuchen wir jedes Jahr sein Grab.
On his death anniversary, we visit his grave every year.

The word is a compound of 'der Tod' (death) and 'der Tag' (day), connected by an '-es-'. So it literally means 'der Tag des Todes' (the day of death).
Der Todestag der Queen war ein weltweites Medienereignis.
The date of the Queen's death was a global media event.
The meaning depends on the context. If it is about remembrance, commemoration, or a recurring date, it means the anniversary. If it is about an investigation or a report, it means the actual day of death.
Zum 100. Todestag des Komponisten gibt es ein Sonderkonzert.
There will be a special concert for the 100th anniversary of the composer's death.
Most often, the word is used with the preposition 'an' (on), which merges with the article to form 'am'. The genitive is also very common for naming the person.
Am Todestag meines Großvaters zünden wir immer eine Kerze an.
On the anniversary of my grandfather's death, we always light a candle.
When the anniversary is meant, the verb 'sich jähren' (to mark an anniversary) is often used. For example, you say: 'Der Todestag jährt sich zum zehnten Mal.' (It is the tenth anniversary of the death.)
Nächstes Jahr jährt sich ihr Todestag zum fünften Mal.
Next year will mark the fifth anniversary of her death.
birthday
'Geburtstag' and 'Todestag' both refer to an important date in a person's life, but their meanings are opposites. 'Geburtstag' is the day of birth or the yearly anniversary of it. 'Todestag' is the date of death or the yearly anniversary of it. The words can be confused because they are built in the same way and often appear in calendars or biographies. But the difference is clear: 'Geburtstag' belongs to the beginning of life and is often celebrated; 'Todestag' belongs to the end of life and is used more for remembrance.

Am Todestag ihres Großvaters zündet sie jedes Jahr eine Kerze an.
Every year on the anniversary of her grandfather's death, she lights a candle.
Heute feiern wir den Geburtstag meiner Mutter mit der ganzen Familie.
Today we are celebrating my mother's birthday with the whole family.