My Espanol is not the best either, but “Una dia” is wrong.
While most words ending in “a” are feminine gender, Dia is masculine. Un dia. With an accent mark, IIRC.
“La mano” [hand] is an exception that goes the other way.
I may have quoted it wrong; article says Un dia
You are absolutely correct.
It is the same in Portuguese.
In Italian, we have ‘la mano’, but for day, ‘il giorno’, so the ending agrees with the gender in the Italian for ‘day’.
Long story short, the protesters are linguistic morons.