How come it’s the Santo António Festival whereas it’s the São João Festival in Porto?
Santo and São – two words to describe Saint?
Why two different names for Saints?
‘São’ when the name begins with a consonant. Even when you try to say ‘Santo’ Joao, the sound itself has an ‘unresolved’ kinda feeling to it! (Note: Post corrected after Joel’s comment below)
I think you meant the other way around, right? Santo António (vowel)
São Jorge (consonant)
And confirmed: https://ciberduvidas.iscte-iul.pt/consultorio/perguntas/sao--santo/8203
This is cool, I always wondered about this but never remembered to look it up!
Yes, that’s the explanation. It’s so weird, this is one of those things I never ever questioned before. I myself am learning a lot with all of your interesting questions - it’s as if I were blind to all the craziness going on in European Portuguese and now you made me keenly aware of it all. Thanks, @kiwijock 
Yes! You are right! That was a ‘Tongue of the slip’! 