What star sign is December 14th?

Sagittarius because that's when my m8s birthday is

^Yes. Another reason it's Sagittarius is because Sagittarius is between November 23 and December 21st.

The date it changes over from Sagittarius to Capricorn varies from year to year but that particular date would make someone born Dec. 4th a Sagittarius Sun. Keep in mind it's only 1/12 of the person's natal chart.