J.K. Rowling gave most of these characters birthdays; Harry's a Leo (July 31), Ron's a Pisces (March 1), Hermione's a Virgo, the Weasley twins are Aries (born on April Fools' Day), Dumbledore and Snape are Capricorns (January 1 and January 9, respectively)...and that's all I can remember from memory.
EDIT: Here's a very helpful site from
Veritaserum that lists the birthdays of the major characters, as well as those who are part of the cast and crew of the movie series. I've copied and pasted just the list of the book characters, though:
I've actually attempted to make natal charts for some of the characters, but this is more difficult than you'd think because J.K. Rowling's timetable is off a bit throughout the series. I believe Harry and Ron were both born in 1980, and Hermione was supposedly born before them although I believe her year of birth is listed incorrectly in the timeline in one of the special-feature DVDs; I believe it was the Chamber of Secrets.
I have been particularly interested in Severus Snape, my favorite character in the series; one of the charts I had created for him--although the year of birth is incorrect if the dates on Lily and James' tombstone in The Deathly Hallows are a correct indication (they all were in the same year at Hogwarts and therefore should have been born around the same year)--had a retrograde Venus, which greatly intrigued me. It seemed impossible at the time that Snape could have ever loved someone, so I dismissed the chart as bearing any relevance on the character, but now I wish I had taken it more seriously...
Arian Maverick