I'm confused. At what point or moment did Yoonsung got his memories back?

After cleaning the lodge, they took that diary to their own home, then he happened to read that. I must be around the time when he pretended to throw the ring into the dustbin

thank youu(⌒▽⌒)

To be more specific, it in chap 41 that he found the diary and suddenly knows hakyung birthday

Thank you so much for this! Reading it again now hits different ╥﹏╥