As it's coming up I want to go ahead and clarify for any newer witches:
Mabon is on the Fall equinox, but the Fall equinox is not Mabon.
Some witches from other cultures may call it other things. Some may not celebrate it at all.
Mabon is known now to be a Wiccan holiday, but in actuality, it is the name of a Welsh god. While it's not uncommon for holidays to be named after deities, Aidan Kelly gave the Fall equinox that name for reasons he mentions in this Patheos post not too many years ago.
Secular, nontheistic, eclectic, or unlabeled witches may just call it the Fall or Autumnal equinox. It's better to ask someone if they observe it as such before wishing them a happy/blessed Mabon.
Wicca's Wheel of the Year was created by Gerald Gardner and other wiccans and takes pre-existing holidays, repackaging them as Wiccan and claiming they're a part of an "ancient" religion (it isn't ancient).
The holidays that were taken from their original cultures are ancient, however. The Celtic Lughnasadh, Samhain, Imbolc, and Beltane, the Germanic Yule, and the Anglo-Saxon's Lammas for example. These were all taken out of context, the cultures conflated, and more due to the appropriation of Gardner and others.
I personally don't care if you call it Mabon, it's just good to know the history of the holidays that you celebrate. I also wanted folks to know why there may be some funk around the use of the name.
My autumnal equinox post will be coming out soon where I talk about a bunch of different holidays and histories involving this equinox for anyone interested!