Zodiac
I know what you're thinking.  "But we never find out when Cedric's birthday is in the first place, so how do we know what zodiac sign he is?"  Okay, this is just my opinion, but based on the zodiac and all that fun stuff I figured a birthday out, so here's my explanation of why I think Cedric's birthday is April 30th.  Of course, this would mean that by the time of the third task he had already turned 18...but hey, why not?  It didn't say one way or the other in the book how old he was at the end, so I'm entitled to say he was. :P
Being born on April 30th would make him a Taurus (April 20-May 20), whose ruling planet is Venus and whose element is Earth (I would have thought Air for a Quidditch player, but it still works...).  Taurus people are said to be determined (yup), efficient, stubborn (I'd imagine him to be so, yeah), cautious, placid, persistent, enduring, introverted (yup), conservative, conventional, materialistic (Cedric?  No...), security conscious (sounds like Moody's a Taurus too), stable (yup), industrious, dependable (very), and one generally having significant financial ability (well, we don't know that much about the Diggorys, but Amos Diggory is in the Ministry of Magic, so...).  Thanks a million to S6 for the happy zodiac descriptions. ^-^ Oh, and the reason I chose the 30th, rather than some other day?  Because August 31st was the day I finished GoF, and I think I started working on this site a few days later.  But since there's no 31st during Taurus's reign, I made it the 30th.
Now how about Chinese zodiac?  Lessee, if GoF took place in 1994 that would make Cedric born in 1978, right? (I hope I got the math right...) Which would happen to be the year of the Horse, specifically an Earth Horse year.  Earth people are hard workers (also a characteristic of Hufflepuff), inwardly focused, and patient but stubborn.  People born in the year of the Horse are supposedly energetic, good with money, like to travel, impatient (?), self-reliant, and people who know how to motivate others, who sometimes feel inferior to their peers, and who tend not to look at the Big Picture.  Their most compatible matches are Dog and Tiger (wah...I'm an Ox and Cho (if you're a Cedric/Cho shipper, which I am not) is a Goat).
I could find a bunch of other zodiac systems and find out what _other_ signs Cedric is, but I'm finished playing Trelawney for now. ^-^