Gwyn's Birthday Can't be in January (or can it?)
The latest argument of the fandom about Gwyn's birthday has reached me and I'm a bit baffled at how people are talking about some things as though they were foolproof facts.
We all know she was conceived in Calanmai. There isn't any discussion about that. In our world, Calanmai is a welsh festival to celebrate May Day and it marks the halfway point between the Spring Equinox and the Summer Solstice. It's counterpart in the colder months is known as Samhain, November 1st.
The first thing I want to point out is that there's a retcon from ACOTAR to ACOMAF about Calanmai, or at very least, some imprecision. That is because in the first book Lucien says verbatim "Fire Night signals the official start of spring—in Prythian, as well as in the mortal world". This does create some ambiguity, as in our world, the official start of Spring is the Spring Equnox, at the end of March.
ACOMAF later introduces us to Nynsar/Starfall, which is even earlier in the Spring, and that's great! Cleared up the fact that the official start of spring =/= actual start of spring. I'm not going to dispute that but rather, alright, Calanmai is indeed May 1st.
As Fae Pregnancies last 10 months to the date would be March 1st. So Gwyn and Catrin are March Babies. Lovely little piece of math there. There's just one problem: this line of thinking assumes their mother carried the pregnancy to full term. Since we don't actually have Gwyn's exact birthday, we can't pinpoint the lenght of her mother's pregnancy and when she was born.
Life isn't rocket science. Every single day children are born way before their due date, for whatever reason, and this is particularly true to twins. Most twin pregnancies aren't, in fact, carried to the neat 40 weeks mark. In ACOWAR, Nyx himself was born way earlier than normal because of complications and after Nesta resurrected him and Feyre, he's mostly fine.
Bottom line is that no, you cannot take it as a fact that Gwyn's birthday is in March just because that matches the usual length of fae pregnancies, since we don't know long their mother was pregnant. Yes, the opposite is also true: we can't know for sure if that she was born in january. Certainty doesn't exist here for either side.
But they could be, if Sarah wants them to be. And that's the point.