Actually, it’s four-leaf clovers that are considered lucky, because they are rare — 1 in 10,000. It’s the three-leaf configuration that is common. The four-leaf clover was a Celtic charm against evil and for luck. read more
I've always thought that's it's because they're so rare- There is only one four leaf clover for around every Six-thousand three leaf clovers. Also, people say they have better luck on the day they find one, which is questionable. It could just be considered lucky because someone decided it should be, and then they had a couple lucky coincidences. read more
Some believe that it’s so hard to find a four-leaf clover that if you do find one you are really lucky. When St. Patrick came to Ireland he used the three-leaf clover to explain the balance of unity and separation of the holy trinity, the Father, the Son and the Holy Spirit. read more