Astrological enthusiasts believe that many different personality "types" dwell within each of the signs. But some astrologers take that concept one step farther, suggesting that your personality may be affected by when exactly in your sign's "season" you were born.
When we talk about the 12 astrological signs we know and love, the wheel of the Zodiac is divided into 12 sections (one for each sign). This represents the (approximately) 30 days the sun spends in each sign. But, in order to determine the specific "types" of each sign, this same wheel is divided into 36 sections known as decans. There are three decans for every sign and each one is ruled by another sign of the same element (yes, a sign within a sign).
Luckily, there's a simple rule of thumb you can follow to determine the decan in which you were born: The first 10 days of your sign's date range make up the first, the second 10 make up the second, and the last 10 days make up the third. If you've ever heard an astrologer say that you were born "early," "in the middle of," or "late" in your sign, this is what they meant.
If this is starting to sound too much like an SAT problem, here's an example. The three decans within Aries are ruled by the three fire signs: Aries, Leo, and Sagittarius (in that order). So, people born under the first decan will be more influenced by Aries and its ruling planet, Mars, while those born into the second may exhibit more Leo-like traits. By that same token, people born in the last 10 days of Aries season may possess a few Sagittarian tendencies.
For the record: Being born in the second or third decan of your sign should not affect which sign you identify with overall. You are still as much an Aries as you were before you learned that Leo ruled your decan.
Instead, you should take your decan as one more element in your astrological makeup. Your decan is similar to your rising, moon, and planetary signs, in that it can reveal a part of your personality that you hadn't considered previously. It should only deepen your understanding of how your sun sign is reflected in you, rather than negate anything about it.
If you're ready to get specific and discover your personal decan, you can find out more here. All you need to know is your sign's date range and your birthday (which we're pretty sure you've got on lock).