this is used at the beginning of sentences for no reason, other than humorous boasting. it doesn't even have to refer to a memory (as it usually doesn't). it's just a fun and silly way to start random sentences. ask my friends, i use this one WAY too much. Example: hey, remember when i was the supreme ruler of the world and had my finger on the button? that was cool.
REMEMBER WHEN...
Meaning of REMEMBER WHEN... in English
Slang English vocab. Английский сленговый словарь. 2012