If you're stuck, be sure to check for hints. In case the hint wasn't clear enough/something broke, you can report this on the novel's discord for fastest response or here.
1. Click the Red Heart in the top-right corner.
2. In the new window that appears, click the Gallery text in the top-left corner.
3. Find a card for the event you want to unlock.
A new, much better event tracker will be added in the next version.