Yesterday was my 39th birthday. I wrote about and said that I was starting my 40th year of life. In my comment section, some people congratulated me for turning 40.
I didn't celebrate my 40th birthday. I'll do that next year.
If you're confused, think of it this way. When you celebrate your 1st birthday, your parents are throwing you a party because you've been on the planet for one full year. That day, you are also starting your second year of life.
So, yesterday I celebrated the end of my 39th year on the planet.
I know this is a revolutionary way of thinking but for those of you that are confused, maybe the above explanation will help you to understand.