Tanya - The event triggers automatically in the Mansion Lobby once you have completed as many events which occur in earlier days - prior to that task - as you can.
Key item - go to the Store in the city. Buy something to defend yourself with.
Any time you get stuck, don't try to focus on where you are stuck. All characters in the Task Journal are interconnected. Their stories overlap and intertwine on occasion.
Thus, if you ever get stuck, look through the tasks for the lowest DAY number and do that, then repeat until every character's tasks are caught up.
The "locked" event may be related to someone else and seeing it BEFORE that other person's event may break continuity/spoil something/confuse you.