I am the luckiest.
I did not answer. Others talked about unexpected free days with nothing to do/nothing planned. My answer has to do with my babies. And this friend who asked the question happens to be walking through a time of infertility. I remember what it feels like and wouldn't want to blindside her. I remember the offhand remarks people made when it took so long for me to become pregnant. (Specifically cutting, and probably something I've mentioned on here before: "you know, Mari, babies grow up," as if I only wanted a baby and didn't ACTUALLY want all that parenting entails.)
It's hard to pin down a specific day, but my happiest times were sitting in my nursing chair with a new-ish baby. I was on maternity leave with Jane, had no responsibilities other than keeping her alive, and I was good at that. I had a great, still-new-to-me back yard view, the temperatures were nice after she was born and stayed cool/comfortable for a while. (Weather affect my mood more than I'd like to admit I think.)
I remember my girls' early babyhood as happy and content times in my life. Livia's is less vivid in my memory, probably because the juggle with Jane and that transition takes up some of the computer memory space of my brain's hard drive. But I know the bone-deep joy from caring for the baby you created, carried and birthed. And that was certainly my happiest time, even with sleep deprivation that I know I was experiencing simultaneously.
Speaking of happy -- I got a lot of happy birthday wishes on social media, mainly Facebook. It STILL kills me that some people write "Happy Birthday, Mary" even though my name is RIGHT THERE. That Indiahoma education for you I guess... For the last 2 years or more I have not written anything on most people's Facebook pages for their birthdays. There are exceptions - immediate family mainly, and even then I probably post ABOUT them and tag them on my wall. (Of course people I am close with in real life get a direct email or text birthday wish!) But seeing the cross-section of people on my life who remembered me (even when it was just Facebook saying "hey, it's this dummy's birthday") made me feel a little happier actually. So perhaps I should resume the ritual of "HBD" on others' walls. MAYBE...