It could be celebrated on December 1st but I think I perfer July 23rd, because that’s the day Altersex was coined.
Altersex is an umbrella term, containing multitudes of sex characteristics, with Salmacian being one of them. So yes, everyone who is altersex will be celebrated.
I don’t believe I’ve seen an Altersex Day before but I do know that altersex person exist everywhere. It’s easy to forget that you’re not the main central of the world, so we must talk about everyone when we say altersex. No matter the gender.