Signs of zodiac are a range of dates under which people are born. In real life there are 12 of them. It is not clear whether there are more, same or less in Arthur's world, or if they fall on the same date ranges. Known signs referenced in the show and those born under them include:
In real life, Libra ranges from September 24 to October 23 according to tropical boundaries, or October 31 to November 22 according to IAU boundaries.
Libras in Arthur's universe may be different from both of these, as in Dad's Dessert Dilemma two days after Galileo's birthday (February 15 in real life) Arthur says Fern's birthday is coming up "soon".
Alternatively, Galileo might be born at a different date in Arthur's universe.
In real life, Pisces ranges from February 20 to March 20 according to tropical boundaries, or March 12 to April 18 according to IAU boundaries.
D.W.'s Very Bad Mood is closer to the IAU interpretation, since this occurs 2 weeks before Francine's birthday and there isn't any snow.
The Friend Who Wasn't There is closer to the tropical interpretation, since it shows Muffy shopping in winter for a gift for Francine.