El doce de diciembre.
You say, "duodécimo/ma."
Posicíon Duodecimo
To say December in "Spanish" you must say "diciembre"
diciembre
diciembre
Coup d'état of December Twelfth happened on 1979-12-12.
December
Yes, twelfth is the correct spelling of 12 in dates.
December is the twelfth month of the year.
El doce de Deciembre.
We begin counting with December 25th as day one, the "first day of Christmas." Counting to the "twelfth day of Christmas" brings us to January 5th, the end of the Christmas season. January 6th is the Feast of Epiphany. Some say this is twelfth night, but would actually be the 13th.
You could say "El último diciembre" or "el diciembre pasado".