Syllva and Irrylath made their pilgrimage in the first place because Syllva was infertile after giving birth the first time. When Irrylath and Aeriel touch down in Esternesse, she's managed to have six subsequent sons. Huh?
It's entirely possible her husband was the infertile one.
And that Irrylath was from, well, a great deal of luck—if it's from low sperm count or mostly defective sperm...well, get enough in there and something will eventually hit.
How long is a daymonth? Itís amazing how hard it is to divine this from context, even given the sheer number of times itís used throughout the trilogy...
It's a lunar day that's as long as an earth month, because the moon takes the same amount of time to rotate on its axis as it does to orbit the earth. This is why the daytime and nighttime periods are called fortnights (i.e., two weeks), and they wake and sleep multiple times per fortnight.