Astrological signs are determined by birthday. Each of the 12 astrological signs represents a different constellation observed and believed to have meaning by the ancient Babylonians.
The 12 signs of the zodiac roughly correspond to the 12 months of the year, though they begin in the third week of the previous month and extend into the third week of the associated month. For instance, although the astrological sign Scorpio is roughly associated with November, anyone born between October 23rd and November 21st is classified as a Scorpio. Following that pattern, Sagitarius roughly corresponds to the month of December, followed by Capricorn, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo and Libra. Those born near or on the cut-off between two signs are thought to demonstrate qualities of both signs.