Both Erinnerung and Gedächtnis can be translated into English as "memory". Are there instances where you would use one word and not the other?
Both "die Erinnerung" and "das Gedächtnis" can be used to express "in memory of somebody/something":
The second sounds a bit more formal, but otherwise their meaning is rather the same.
"Das Gedächtnis" can also be used to express "the memory":
Here, "Erinnerung" would not fit well.