What Does the Heart Mean Next to an Event?

I went back to my Main Save, as I needed a break from Cottage Living. I deleted some of the saves back up to their Third Term of Uni, since I rushed the game trying to get to a certain point before the new EP was released. Silly me. I just discovered something very interesting about this new calendar. I clicked on the Graduation event, which opened up on the screen left panel. It not only gave the times of the ceremony for each of my seven Sims, but, on the left side of the screen was a blank heart. I found that by clicking it (just trying to understand what it meant) it would fill in.

What does this mean? What is the significance of it?

Please, and thank you, in advance.