A good question
I think there are a few, picking one is not really something I can do, as each of the days have their own special meaning to me and no particular one defines me as a person.
Surprisingly the day I found out I was PG was not the happiest. Yes I was happy, but it was mixed with so many other emotions and questions, it took me longer to become truly happy with the concept of a baby growing inside me. Planned as baby is, I was still shocked as we conceived so quickly, it was hard to take on board.
I'm also not going to say my wedding day, as both hubby and I were and are very low key about that sort of thing. We both wanted as little fuss as possible and to just go back to our lives without anything changing. We don't wear wedding rings and I kept my surname. It was our day and we shared it with 4 other people, but it isn't our defining moment in our relationship. But it was special and shall stay with me always.
I'm going to list other things instead. Things I can look back on and feel the memories. In no particular order
* A day in September 2004 - Getting off the plane at Adelaide airport in Australia after 30 plus hours of travelling and seeing my Bloke waiting for me with the biggest smile on his face. I felt like I had come home and never wanted to leave. All our long distance relationship problems melted away there and then and we've never looked back. The cab ride back to his house we didn't speak, just cuddled. He smelt wonderful.
* Leaving England and moving to Amsterdam. I spent many wonderful years living there and am eternally grateful I had the chance to spread my wings and live overseas, there and elsewhere. It gave me a whole new perspective on life and how to live it.
* Me and my dogs, one early autumn day in blazing hot sunshine walking over the Quantocks for hours. I spent many happy days just the dogs and me, but that one stands out. My shadows and me.
* The day I saw my first ever Cuckoo. A strange one I know but after 30 odd years of waiting, it meant a lot. I followed it for hours. Was a great thing to see.
* When I finally admitted to myself that before I could truly love someone else, I had to be able to love myself. It meant I was able to finally embrace my relationship and the person. It was bare bones and harsh, but it gave me a whole new life with someone I love more than anything. I never thought it would be possible to share what I share with someone till I met my Bloke.