"Mi cumpleaños es en diciembre."
The months aren't capitalized in spanish, and there is a little squiggly like this ~ above the "n" (ñ).
There are also a lot of great online translators that can help you. Try these...
freetranslation.com
Google.com/translate
your answer is in your question...............
my birthday is on december 31st
December 31, 1869
december,31
December 31, 1869
December 31, 1491
December 31, 1969
december 31 in a picnic basket
December 31, 1984
Siné was born on December 31, 1928.
Odetta was born on December 31, 1930.
Are Kalvø was born on December 31, 1969.
Jorjão was born on December 31, 1970.
Are Kalvø was born on December 31, 1969.