What does the green check marks on my calendar mean?

I went to my calendar to see if I would be on my period on my SO birthday and I noticed that somedays on my calendar have a little green check mark in the bottom right corner and other days have a little pink heart in the bottom right cornor. What do the check marks and hearts mean? I thought maybe the hearts where for the days I'm on my period but their not just on those days! Please help! I'm so confused!