Hmmm... This one makes me think.
I would say:

-The day my grandpa died
-The day I met my best friend
-The day I learned my alcohol limit
-The first time I used cannabis, also the day I first researched it and realized that my government had been lying to its people for decades
-Going away to college
-Realizing that my ex and I were polar opposites
-A spiritual experience after eating 'something'

and others. Life is an ongoing journey, so I find it hard to find single individual events that shape me. Every second of my life has had some sort of an impact on who I am.