Actually there are 13 constellations that the sun passes through. Ophiucus, the Serpent Bearer, is ignored by astrology. It makes up a little over half a month. Ignoring Ophiucus only serves to make astrology more inaccurate (is that possible?).
Source(s):
I took astronomy. And of course to back me up: http://en.wikipedia.org/wiki/Zodiac#Table_of_dates

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Here's what Wikipedia says about the thirteenth sign of the zodiac.

"Although Ophiuchus intersects the ecliptic, it is not a zodiacal sign in most versions of astrology. The signs are defined as 30-degree segments of the ecliptic, of which there are twelve, and they are named after nearby constellations at the time the system was developed, rather than being defined by the modern constellations. However, a few sidereal astrologers consider the Sun to be in the sign Ophiuchus when it is in the constellation Ophiuchus, which as of 2008 is November 30 to December 17."

"Although not incorporated into the 12-sign zodiac, Ophiuchus and some of the fixed stars in it were sometimes used by astrologers in antiquity as extra-zodiacal indicators (i.e. astrologically significant celestial phenomena lying outside of the 12-sign zodiac proper). An anonymous 4th century astrologer, often known as Anonymous of 379, seems to have associated "the bright star of Ophiuchus", likely α Ophiuchi, with doctors, healers or physicians (ἰατρῶν)."

Basically the sun passes through it, but mainstream astrology doesn't recognize it as a true zodiac sign, because the sun only passes through it for 19 or so days.

Following the Wikipedia link to Sidereal Astrology you can learn about the differences between Tropical and Sidereal Astrology.
Source(s):
http://en.wikipedia.org/wiki/Ophiuchus#Astrology
http://en.wikipedia.org/wiki/Sidereal_astrology

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There isn't a 13 month, but the 13th sign is Ophiucus 11-30-12-17. The 13 signs divide up the 12 months of the year.
http://www.geocities.com/astrologyconstellations/ophiuchus.htm
Source(s):
http://www.geocities.com/astrologyconstellations/index.htm
http://www.geocities.com/astrologyconstellations/ophiuchus.htm

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