As far as I know this assignment is done by a HTMLCard's javascript function.
C:\Users\[USERPROFILE]\Documents\eXtreme Movie Manager 8\CardsRepository\js\zodiac.js is the file you have to edit.
I don't know the dates so I can't correct them but the format is really easy to modify:
c is the month and b the day. Maybe these dates are wrong.
if (c == 3 && b >= 21 || c == 4 && b <= 19) {
a = l("Aries") + " - #x2648;"