Well, congratulations on the grand baby!!!! Fabulous news!
This precious little one is going to be a Libra. You'd probably enjoy reading about this sign
here rather than see me yammer on about it. There's a lot on that site and it's usually pretty decent. (Edited to add: Some astrology sites have Sept 23 as Libra and some have Libra starting on Sept 24. I've always thought it was Sept 23 so that's what I'm basing this on.)
Libras are usually thought of as kind, intelligent, compassionate and some are very empathic. The not-so-great thing about them is that they can sometimes allow themselves to be walked on. It's a good idea to foster a strong self-image
early in these wonderful, loving kids to help them stand up for themselves. They usually don't have a problem standing up for others, though. It's just a Libra's general nature. It stands to reason that Libras also make wonderful parents as well.
Being actually born ON Mabon would strengthen this child's natural fairness and balance, in my opinion. I'm not an astrology expert by any means, though, so these are my opinions and beliefs only.
But again, I have to congratulate you on this wonderful event. I've ached on and off for another baby, although mine are in college now. Hubby tells me that I should just be at peace with it and wait a few years because legend has it that grandkids are WAY more fun!!!