For me there was, for example, my 20th birthday,
...when I was a student in Paris and Mme. Vidalenc, the woman in whose house I was renting a room, threw me a birthday party with chocolate whipped cream cake which I ate it even though I don't much like chocolate cake or real whipped cream - I only like the fake stuff from the can or Cool Whip - and champagne which I drank even though I didn't drink and afterwards I threw up all night long.
I also remember my 33nd birthday, the night before which I'd been up all most of the night with a crying new-born, a sick 2 -year-old and a sick 4-year old, and thinking I felt more like 83 than 33.
On my 62nd and 64th birthdays I was in Spain with Tom hiking the Camino de Santiago de Compostela.
And I expect this past birthday should also go into my basket of memorable birthdays, maybe not so much for what I did on that day as for the fact that the day marked the 65th rotation of the earth around the sun since my arrival on the planet, officially signifying in my society and culture my entrance into a new phase of life as well as bringing some changes in my legal and social status.
However I did in fact end up spending my 65th birthday involved in something memorable: on that day I was, in a small way, part of the 2016 Presidential campaign.